well if mice are anything like rats regarding tumors, it sounds like she has a mammary tumor. Our rat, Oreo, got one at 16 mos old. She lived for 5 mos. It got extremely large and made it very difficult for her to get around. We put her to sleep on April 1. I don't know what the timetable would be for a mouse though. We decided to keep Oreo going until she seemed unhappy and miserable. Generally mam tumors aren't painful, but they get very large and make life difficult. I guess if they get entangled in internal organs they would be. So we watched her. We decided it was time when the tumor got hard (tumors like this can rupture) and when she was hanging her mouth open to breath. She started breathing with her mouth open on 3/31. On Apr 1 the tumor seemed very hard. I just couldn't bear thinking it would rupture. What a horror that would be for her mostly then for us. Oreo's tumor also popped out of nowhere. The vet said that they often do that.
So sorry about your mousey