Go and buy a $10 pressure gauge from a hardware store, and hook it up to your washing machine hose bib.
Then see what your house water pressure is.
Next, turn on your bathtub faucet to full hot for a couple minutes, to partially drain your water heater. Shut off the tub faucet, make sure no one in the house runs any water for a while, then immediately go and watch the pressure gauge. If it starts to rise under these circumstances, you have "thermal expansion" and need an expansion tank.
I am guessing that your old toilet may have allowed this excess pressure to vent off from time to time so the TPR valve didn't leak - until you replaced the toilet.