In all honesty, I do doubt you are pregnant. Those symptoms can happen for other reasons than pregnancy and tying of your tubes is pretty permanent. There are some cases of women having either the clips come undone if you had your tubes tied that way or scar tissue forming in such a way that your tubes sort of refuse if you had the tubes cut. However, the incidence of tubal pregnancy with either of these situations is very high. At 50, you can theoretically get pregnant. Staying pregnant is another story as egg quality is quite low by 50. It's newspaper worthy for a mother to have a healthy baby full term at 50 plus unassisted with fertility treatments. But never say never. I'm not sure what you mean by inconclusive pregnancy test. Those are pretty dead on. Do you mean you left it sit awhile and the second line appeared? That happens due to evaporation and pregnancy tests have to be read within the time frame the box tells you. So, maybe I'm not understanding what you mean inconclusive and I'll be happy to help you further if I can.
It's possible to get pregnant, even if your tubes are tied. Given your age, it's no likely- but you are still having a regular cycle, it sounds like it's a possibility. Did you end up taking a test?