1999 Dodge Caravan problem with engine running hot

Last winter was extremely cold, and on my way to work on one of the colder days, my check engine light came on.I heard a "thunk" from the engine, and after that the check engine light never came on again.

Now, this summer I've been noticing that my van has been running hot ( about in the middle of the temperature range) and the coolant return hose to the radiator when the car is turned on normally I could feel coolant moving through it and NOT be able to pinch the hose, now I can pinch it almost together and dont feel anything moving. Both my radiator and overflow are full of coolant. I'm wondering if this could be as simple as the thermostat not working correctly or if its the water pump. How would I go about checking to be sure which it is?