I Have Vein That's Swollen Over My Left Knee Cap.

I have vein that's swollen over my left knee cap. I'm a 22 M and I'm very physically active. I noticed it during MMA class when I was drilling my penetration steps, which is like dropping the front knee down to a kneeling lunge, slide the back leg forward, and repeat. There's no pain, just uncomfortableness. I was mainly just focusing on stretching my legs and wasn't forcefully slamming my knees on the mat. What kind of issue can I be having and what can I do to treat it?