I smell fishy, but only when it's hot. Help?

2 years ago, I wore the same denim shorts 2 days in a row while camping, and it was really hot. Later on day 2, I noticed a fishy smell. I thought maybe I'd gotten a yeast infection or something. For the rest of the summer, I smelled it every time I was sweating down there. It went away after the summer, and I had started taking probiotics with cranberry. I can't remember if I had the problem last year. Today was the first really hot day of the year, and I wore denim shorts and sweat a lot. Took off the shorts when I came home, and the smell is back. I'm not currently sexually active, so I know that's not it. Does anyone have any ideas of what my problem is and/or how to fix it? Should I take the probiotics again?

I appreciate any input, but please keep in mind that I wouldn't be posting this if I could afford a doctor :(