Why is my dog acting strange?

My dog (5 year old Lab/Rottwieler mix) had a fight with a possum 2 nights ago, And since then she's been acting weird. She keeps biting and licking a lot, and she will just randomly turn around and look at her backside like something's biting her, and she has her tail down when she walks. And we thought maybe she had fleas, because she's had fleas before and she acted like this when she had fleas last time. So we put some flea medicine on her (or tried to) and then we put her outside and she was running around crazy, and then she ran, and just sat down. and she wouldn't even come for a treat. WHy is she acting like this? Is it fleas biting at her?? I'm just worried.