M.R.
I know people who were conceived at least a few years after their moms had tubal ligations, so it is possible.
If you don't have insurance, look in your phone book and see if there are any area pregnancy centers, either crisis centers or public health clinics. Here we have several centers that offer services to women who are unexpectedly pregnant, including testing and early ultrasound. If you continue to have symptoms, try another home test (sometimes hormone levels are lower even with a normal pregnancy). I believe a tubal pregnancy sometimes might not have a positive test. Either way, if you miss another period, definitely find a clinic.