It doesn't sound like bipolar to me at all either. From what you describe, it really sounds like something far simpler.

A lot of times, people can be very agreeable and sweet and (insert myriad other wonderful qualities here). And indeed, they really are. But it's not at all unusual that some of this is accomplished by stuffing things down. In the interest of being nice. Bottling things up will eventually lead to periodic kabooms! Over something minor. It's not that they do it intentionally, in fact it may be so reflexive that they don't even realize they are doing it. It can be a really ingrained thing, but can be unlearned with help from a therapist. It'd basically just be learning to be assertive enough to know what you feel matters, it is ok to speak up, and that putting absolutely everyone else first isn't always the virtue it seems.

If this is what's going on (remember, we can't diagnose or anything), it's quite. unlikely meds would even be used, as it wouldn't be a condition to treat, just learning a better coping method.

As far as getting help, I'd go with a therapist, not a psychiatrist. Aim for a "good fit". A lot better and quicker progress can be made when working with someone they are comfortable talking to. If that's not accessible for awhile, there are books that can help. Sorry I don't have any recommendations, but others might.

Good luck, and remember, it's not about getting her "fixed", it's about working together for her to be more comfortable expressing smaller frustrations in the moment instead of letting them build up.

(P.s., and not that you asked, but crying is not a bad thing for men. There can be a lot of stuffing down involved in that too, so there may be something in there for you too. Win-win! )