A swollen leg could be caused by many different conditions so it is hard to say. It’s always difficult to diagnose conditions over the internet as someone would need to do an examination of your leg, do some tests and look at your past medical history to get a full picture of your situation.
At this web site we specialise in STI and one thing I can say is that a swollen leg would not be a general symptom for an STI. As most STI do not have symptoms, I would recommend getting an STI test if you are concerned that something may have passed to you.
The best thing would be to see a health care professional in person about your leg.