Wow.
If the drummer said he was cool with it, is there a reason for you not to take him at his word? Is there a history of cheating on his girlfriend?
Also, avoid all situations where the two of you are alone.
I think you must let your boyfriend know, it is only fair. But only if he can act maturely and not drive over to the drummers house and punch him in the nose. You owe it to your boyfriend to let him know someone made a pass at you, but you handled it, and it's cool and your boyfriend is still your one and only guy. But again, only if he will not act rashly.
As it sounds, the drummer made a mistake, He read things wrong. However I wonder about his maturity as the first thing he did was blame you for putting out signals....
The apparent lack of maturity on his part is why you should not allow yourself to be alone with him. Show up 5 minutes late to every practice and gig from now on. (late to gig meaning uif the band is getting there at a set time, not 5 minutes after the gig is to start)
Also, you said nothing on how he hit on you. Some guys are shallow thoughtless clods and if his delivery is what is creeping you out, I can then understand.
You should have no reason to beat yourself up. You seem to have done nothing wrong except innocently accepting a ride from a friend that you trusted.