Fish staying near top of tank

I have 3 goldfish that I've had for a little over 3 years in a 20 gallon tank. Two of them are male and one is female. I just did a partial water change yesterday and today my female goldfish is hanging around the top of the tank, not gasping for air, but a little part of her is out of the water. She'll go under and swim around for a bit, but then she'll go back to the top and just hang around there. And she seems to stay near the heater. Should I be worried? Is she sick? Both of the other male fish seem perfectly normal.
Also recently, one of the male fish has been chasing her around the tank and really bothering her and she has a tumor on her side (has had it for a year and doesn't seem to bother her much, though if there's a cure I'd love to hear it). She has lots of energy and still has a good appetite. I don't know if this matters but 3 days ago I found their glass thermometer broken (the little balls fell out but none of the red stuff got in the tank) and I quickly sweeped up all the pieces, so I don't think they accidentally ate any of it, but could that be the cause?

 

First off, what are your tank specs?
I don't think there's anything wrong with your female...but the thermometer breaking is worrying. Was it in the tank? I know people don't use mercury anymore, but there are probably a host of other chemicals in there.
I think the male starts to chase the female in anticipation of breeding (prob want to check this) but there is a point where your female will die from stress, so if it becomes too much, get a breeder/qt box in your tank to separate them.

 

Aquarium thermometers use alcohol. The female may be stressed from the male chasing her or it may be related to the tumor.