his secretary spends the night before he days he wants a div

okay do my hubby and I have been separated for almost 4 moths...a few weeks a ago closer to a month he said that he would do watever it took to make the marriage work. I simply said stop drinking, he is a full blown alcoholic. so weeks go by he is doing great not drinking we are talking every night and spending alit of tiome together, he was spending family time with me and my son. Then somehting interesting happened, a friday night i asked if he wanted to hang out because my son was with my om, ya know just the two of us, he himmed and hawwed and said that he had plans to go someone, i got a feeling in my gut that something was up, then I do not hear from him until tuesday when he proceeds to go next door to his sister in which I was there and cracks a beer open in front of me, I knew then that the inevitable had to happen, well he says that he is tired and going to shower and go to bed, within an hour he had showered gotten all dressed up and left there without saying good bye to my son, then get a phone call, yes a phone call, that he is not inlove with me that is is going to get his stuff the next day, i find out later that his secretary at works car had been out front since wed night, he has proceed to spend the night at her house as well as she sleeps over there often, his sister who is one of my dear friends asked him what was up with her, and he says we are just hanging out, i see nothing wrong with that. I know that she has been after him foryears just by her strange behaviors. Now I am worried that this may have been in the works or an affair was going on before. What should I do? I have a dr appointment just to be tested for a disease or something but i knew in my gut along time ago that things were strange with her noway, am I assuming too much or do 2+2=4 in this situation?
any advice what to do from here is greatly appreciated.