All relationships go through ups and downs, but if you get the feeling something is wrong, that things are not as they normally are, look for these classic signs that he is cheating on you. By themselves they may be insignificant and may well have perfectly innocent explanations, but many people whose partner cheated on them, when they looked back saw glaringly obvious clues. When he is with you and his cell phone rings, he sends it to voice mail or gets up and leaves the room to answer it. He takes his cell phone with him to the bathroom.
If you go near him while he is on the phone he turns away from you. He deletes text messages as soon as he sends or receives them; and he deletes his recent calls, incoming and outgoing lists, 'to free up space'. He acts secretively about his computer. If he deletes his emails and clears his cookies you have to wonder why. Your home telephone receives wrong numbers and hang-ups when you answer. His appearance matters more to him than it used to; he shaves more often, uses a new upmarket brand of shower gel or aftershave, gets his hair cut trendily, dresses more tastefully, works out and goes to a tanning salon. He suddenly enjoys different music which may indicate the influence of someone else. He discusses subjects which previously did not interest him. You smell different fragrances on his clothes, for example cigarette smoke and perfume. He seems distant and preoccupied. He spends more time away from you, either with hobbies / interests or his friends. When you ask after any of them he keeps his answers short, without elaborating. He no longer goes shopping with you or takes you out. He avoids being alone with you.
Mutual friends say they rarely see him. He has a new circle of friends, male and female, you have never met before; they may be her friends. His excuses are not in keeping with his previous behavior. For example, at work he always did the bare minimum, but now puts in a lot of overtime. Or he starts having car troubles when his car is still new. Or previously never close to his family, now he visits a distant relative. Your sex life changes. It may become quieter with him just going through the motions. Or sexual activity may increase with him wanting to try out different things, perhaps things he is doing with his new partner. The same with his kissing style. He seems uncomfortable when you get in his car. Maybe she adjusted the seat or left something behind, including her fragrance. He treats you differently. He talks to you reluctantly, even about everyday things, and makes minimal eye contact.
When you ask an innocent question, he flies off the handle and accuses you of trying to run his life, shouting, "Why do you need to know everything about what I do." He picks fights. He criticizes, makes derogatory comments, points out your faults and compares you to others, for example, "Why do you not take more care of yourself like she does." His body language changes. His stance used to be quite open, but now he subconsciously keeps you at a distance by standing further away, crossing his arms, and tightening his lips and eyes. He becomes evasive. Things he used to like about you now bother him. For example, he admired your laid back attitude to life, but now sees you as unambitious; he loved your sense of humor but now finds it silly; He enjoyed spending most of his time with you, but now finds you possessive. He talks a lot about a new colleague or friend.
Hearing her name once is no cause for concern, but when he repeatedly mentions it, take note. He describes a new female colleague as ordinary looking, but when you meet her, she is stunning. He accuses you of cheating. His credit card bill is higher than usual, especially if he has been away on a business trip. He is short of cash, and asks you to pay for more than you normally do. He secretly opens a new bank account. He withdraws more money than usual from your joint bank account. Some of your friends avoid you or change the subject if you mention his name; they may know something you do not. Sometimes we avoid looking too closely at our partner's behavior because we dread facing reality, and understandably so. But if you sense something is off, trust issues will emerge and your relationship will deteriorate anyway. Perhaps start by discussing your fears with a trusted friend. Before accusing him, check your facts and if you can, get proof. Talk to him and let him explain or come clean. Then create time apart from each other, and decide what to do next.