One reddish painless bump on vagina lip. is it an std?

Not likely: Painless bumps are less likely to be an STD. STDs usually cause painful bumps or a vaginal discharge. The only way to know for sure is to have it checked out by your Gyn. Best wishes!

See a physician: to make sure it is not skin growth or sore. STD will also be excluded at the same time, like syphilis

Difficult to know: Without examining you, taking more history, it will be difficult to conclude this is STD.