This happened with my DD at the same age. We had her in a plain, flat-bottomed baby tub lying on a terry cloth sling. One night she freaked out in the tub for no apparent reason that we could think of. She did the same thing for the next two nights. The fourth night, I sat her up in the tub. She started to wail, then I pulled out some bath toys, and it distracted her. She immediately attended to the toys, forgot whatever it was that was distressing her, and she's been fine ever since. She's 9.5 months old now, still sitting in her baby tub and we never did figure out what caused her to get so upset. The only thing we changed was that we sat her up and put toys in the tub.
Nic