I know this pain *all* too well!

Sounds like you need to give your dentist a call. I'm shocked (s)he did not tell you a crown was needed. Let them know you're in pain; perhaps they can put in a temporary and prescribe an antibiotic. Because there is pain there could be an infection at the end of the root. If the cost worries you, ask them the cost, although if you come in and just get x-rays done and everything it should be around the area of $50. I don't know your dentist though, so it may be different.
A crown is normally recommended on root canal teeth to prevent them from fractures, to which they are easily susceptible, but pain means there's something else there.. *maybe* an infection that should be treated with antibiotics. And, of course, the filling will need to be redone. Some doctors will redo a composite for less $$.. I know all of the doctors I work with do so. I hope you can work something out, feel better!