Sounds like you are getting good pumps. The "bulge" you are seeing is due to the muscle being filled with blood. It's temporary and goes away pretty quickly.
You don't notice this "bulge" when you are not lifting, correct? I would assume not. That's because when the muscles are at rest you don't see the "bulging".
And if you are seeing a tricep muscle you should count your blessings. Most women don't get to experience seeing a nice tricep muscle when working out. Sounds like you are pretty lean and your body fat is low. It's all good.