not being able to pump/feed all day

I'll be away from my baby all day tomorrow in the city and at a baseball game in the evening and I'm worried about my milk supply since obviously she'll be with my mom so I won't be feeding her personally and I won't really be able to pump while I'm out and about. Will my milk supply go down and how will I be able to relieve my breasts if they do become too full?! I'm very worried. Also this is the first time I will be away from my baby for more than two hours she's 10 weeks old!