I feel pregnant but keep getting negative home preg tests. Explanations?

[Jessica]

I have three children so I definately know what it feels like to be pregnant. My last period was April 15th. I have tested several time but all are negative. My tubes have been tied for 3 years and cycles have been off but never felt like this before. My tubes were not cut, they have clips on them. I talked to a nurse at the hospital and asked if it was possible for me to be pregnant, and she said it wasn't "impossible". I really feel like I am pregnant but if I can't get positive test, how am I going to prove it? I don't have insurance right now so it isn't as simple as going to the doctor and asking for an ultrasound...help