Am i bad person?i feel terrible............?

well i hope she doesn't read this or know someone at your gym. Other than that, having cramps and a headache is a legitimate excuse and she should understand. You can't make everyone happy by holding their hand for everything. I have backed out of situations that were similiar myself. She probably won't be too mad but might stop asking you to go with her places. It is a little rude to people when they may have counted on you for support to back out on them. I hope it all works out for you both. I am not sure if your basic question was " am i a bad person" if so. No you are not. You are human and humans make mistakes or changes in plans. Relax.