If you are being shafted financially, then you should try and at least see out your current contract – after all, you agreed to go out on the job for the rate you are being paid.
After that ask for more or go elsewhere.
I don’t know the poaching time limit – i have been told various figures over the years.
Ask the agency that you are currently working through and they may get the hint that you are considering jumping ship, so to speak. If you are worth the extra money, maybe they will try harder in the future – if not, then pitch yourself at a higher rate with someone else.
Good luck