My son just turned 6 months and started throwing fits at 3 months. Or earlier. He's generally very good but has a temper to rival that of any five year old I've ever seen!
She could be teething, because my son started doing the same thing about a month ago. I found that around bedtime he started to get really grumpy and read up on teething...that could be the cause. We gave him Tylenol for a few days (as directed by his doctor, of course) and this seemed to work like a charm. I made sure it was teething though by giving him a teething ring or a frozen washcloth (I wet the tip of one of his and put it in the freezer for a minute) to chew on. When he bit down on them like there was no tomorrow, I knew that's what it was, tried the Tylenol, and it worked.
He still throws fits though for other reasons and I think it's really him just trying to explore his emotions and our reactions. He will do the same thing (did just now) before a nap, and I will check on him, pat his back, tell him it's ok and just let him cry it out. I found the more I babied him and picked him up, the worse he got about throwing fits.
Good luck and let us know how it goes!
PS...I just read the response below mine and they are right! We have had our son on a schedule, and it took a few weeks for him to get used to it, but we do pretty much the same thing every day at the same time. Now, if we don't give him his bath by a certain time, HE gets anxious for it. It does sound mean, but just because they are infants, don't think for one second they don't know what they are doing! I know a lot of other people w/babies that have no schedule, what so ever, and those babies seem more in control of the parents than the other way around. We vary his routine some a little to make sure he knows it's ok to be different every once in a while, but it really does work like the other lady posted.
Sorry to be so long winded. :o) Of course we don't know if you've got a schedule or not, just saying it works like a charm for us!