Abnormally long period... FREAKING OUT

So, I'm now on day 9 of my period. 11 if you count the spotting for two days beforehand. And yesterday, I thought I was done. But today I have almost brown, but still red discharge- so I think it's still my period. I had my second baby almost 5 months ago and I just stopped breastfeeding all together recently, so I haven't had a period until now. Before I got pregnant with him, though, my periods were never more than 4 or 5 days. The thing is, my husband called for a divorce about a month ago and I got drunk and had sex with someone else. But now he's wanting to work things out, he doesn't know that I had sex with anybody else, and I'm terrified. Could this be an STD or is it normal to have a weird period like this after not having one for so long? I don't remember this happening after my first son.