If she's in heat, chances are she's 'marking' because they do tend to have to pee more when they're in season. Think of it as advertising! She's putting out 'for a good time call' flyers in other words. Not a big deal. Or it could be new place and stress, or she's not totally housebroken, take your pick.
You can book her in for a spay in about six weeks as far as I know, that'll help and be less messy for you.
I wouldn't use the bells or teach that sort of cue personally. My guys have a dog door most of the time or I make a point of letting them out often, but it's on MY schedule mostly. If I see them acting like they need to go I'll let them out, but otherwise I dread the dogs who scratch or whine or bark to go out and in. Where I work there's a beagle who was taught that way, and it gets annoying sometimes when I'm busy and he wants upstairs, downstairs, wants out, in, cookies, whatever. There's two twins that are crawling and so the stairs are blocked off, and the beagle will bark or scratch at the worst times, then give me a dirty look and do it again, let him in/out and two seconds later, he's at it again. I tend to end up ignoring him because at times he's wanted upstairs, downstairs, upstairs again, outside, inside, downstairs, upstairs, scratching at the wall?..... all within 20 minutes. I'm not there to be his personal attendant, I have other things to deal with! So he's starting to clue in to that a bit. I may bring him some bones to keep him occupied and out of the way too....