Welcome Dazed -
You have found a safe place to ask questions, get support, and figure out how to navigate this shitstorm your life has just become.
None of us ever plan to be here, yet you can see we are over 40K strong.
You don't offer a lot of info, and hey that's fine, but a few thoughts, and my Newbie to do list is to follow.
How long was it, who was it with, and do they have a spouse, if so that other spouse has every right to know what happened, and you can and should tell. Don't tell H you are going to tell, just do it. These things thrive in the dark, and fantasy land the sooner you can drag this thing into the daylight the sooner all the ugliness will be revealed, and things will end.
Read in the healing library over there to your left. There is a ton of useful information, I will also bump some thread that have bullseyes please read the first parts of them, they have helpful information to give you an idea of where to go from here, and How to stay sane doing it.
Now you probably are completely overwhelmed with grief and pain, and that is normal. You may even be thinking or asking yourself what could I have done to prevent this, and how do I fix it. Don't torture yourself. Your H's choice to have an A, is exactly that HIS choice. It's about his need for added attention, ego boosting, and poor boundaries. More on that later.....
Right now you need to focus on you, are you able to sleep? Eat? and stay hydrated? If so great, keep up the good work, if not then you need these basic 3 things to even begin to form rational, solid thoughts. So if you can't call your Dr, and ask for a little pharmaceutical support during this time. Some of us have to take antidepressants (ad's) and others require antianxiety meds to get through this. IT's ok. This quite possibly the most traumatic even of your life.
If your H admits to an A, then you have to take into account that he is probably lying if he says he used protection, in any case I strongly suggest you get STD tested, and yes the full deal, pap, and blood work. Do NOT allow yourself to be intimate with him again, until he has been tested, and has written proof of being clean.
Next no matter what you decide to do, know that you don't have to make a decision today, tomorrow, or next week or next month. YOU DO need to see an attorney, and I tell all new members this. Find out what your rights are, his obligations, and responsibilities are should you choose to S (separate) or D(divorce).
You need to figure out if you want to try to make it work and R (reconcile) your M (marriage). If you are leaning toward R then you need to figure out what you want and need from him for it to happen, and be very very clear what those things are, and that not doing them has severe consequences that you absolutely have to stick to.
Whatever choice you make is yours, and we will support you on that. However please know that the things that you think you should do, and want to do, are most likely the things that allow him to either continue, or not learn from. No one saved their marriages by nicing their spouse back. This is a long tough thing to get through, but you will.
Keep reading, keep posting, and remember you did nothing to deserve this, and you are worth much more.