Can a painless pea size swollen inguinal lymph node be a sign of a std? if so which ones and how common is this? anything to worry about?

Depends on other : symptoms, and exposure to STD's from your partner(s), and whether or not you have been tested for any of them already. Also if there's any accompanying chills or fever. It does not necessarily by itself mean that it is an STD. So call your GYN, and have it examined in person. That will alleviate any worry.

Could be an STD: A pea size painless swollen lymph node in the groin can definitely be a sign of a STD. If you have unprotected sexual intercourse, you should be evaluated for STD. Consistent condom use is the best protection for STD.

Enlarged lymph node: a swollen inguinal node could be from any cause of irritation or inflammation (and infection) in the surrounding area. It could also be a residual node from a past irritation. If there is the possibility of having a STD, schedule an appointment with your doctor for testing and treatment if needed.