Well I'd probably just explain it like that. That the happiness is so welcomed after so much depression that it's hard not to run with it a bit. Really who wouldn't? I mean it's an Extremly nice change.

Also maybe it would help if you dissected you mania or hypomania a little more. Is there a build up, climax, duration? I know for me I will get hypo and this can last for a few days. Either from that point I will continue to climb into a longer lasting manic state where I am quite aggitated physically an mentally. Hard for me to even speak, I stutter and can't even carry a conversation hardly ( those are just a few obvious signs for me) Or the other way, I will get hypo for a few days and then it will dissipate back into a more stable mood or depression. But knowing what exactly it looks like for you can help you and them better judge what is going on.

Maybe you can explain to them too just a little about how you feel when every mood change is questioned an how that effects you. Hopefully they will hear you and relax a little. It also might show them that you are aware and you are watching yourself, so they might be able to ease up a little. I hope that helps a bit, and good luck.