I completely understand how you feel. Although I'm still on maternity leave I absolutely LOVE getting out of the house & away from Moo when the opportunity arises! I've only been able to leave him a handful of times & it's just been the best thing to have a cuppa with a friend or see a movie without any interruptions. I've only ever left him with DH which means I'm not worried at all & enjoy my time "alone". I think Moo gets bored with just me too so we all win!
Don't feel guilty, sounds like you're doing a great job. You obviously trust who she's with & know that's she's having a great time with her Nana.