HELP.. excuse to get out of the house to hobby needed

For about 6-months now I have been living with my GF. She has no clue that I hobby or have in the past but we have a strict don't ask and don't tell policy about our pasts. Well, to this point I have been able to get out of the house under the rouse that I was going to the Martial Arts Gym. Sure, it is embarrassing to show up to a session wearing athletic shorts, sneakers and a T-shirt but to this day I have had no complaints from the providers. Besides the clothes don't stay on too long anyway.  

Later this week I have a 60 min session booked with a provider. The appointment time does not line up with my Jiu Jitsu Class schedule which my GF knows all too well because she used to train there. I need to come up with a clever excuse to free up some time by thursday.   Any ideas?