If the mass is that large and you're just noticing it and you masturbate at least once every two weeks, it's probably just a cyst of some kind. I've never seen herpes, but if you have it you probably would know what a herpes lesion looks like; if the mass doesn't look the same, it's probably not herpes.
I've had a small cyst in my labia minora and sex of any kind only makes it worse (the friction). In addition to abstinence, try sitting in a sitz bath for 20 minutes 3-4/day and applying warm compresses to the area. That may shrink it if it's a cyst. Wear cotton underwear. Don't touch it to keep assessing its size--you'll just aggravate it. Take ibuprofen. I agree with the other poster--it's probably just a cyst, so call your GYN on Monday.
It could just be a cyst. See a doctor, sweetheart.