2003 Nissan Sentra Overheat

Hey guys, I have a 2003 Nissan Sentra 1.8 Liter that is overheating without any true known cause... all symptoms said to change thermostat however after doing so I had the same symptoms and then it blew my radiator. What's funny is it is not always getting hot but getting colder also and staying colder... the only time I have heat in my car is when it gets colder. I also noticed that my reservoir is getting full and overflowing. I know that coolant is flowing through because both upper and lower hoses are getting hot, however the heater core hoses are not.
anyone have any suggestions?

update... running it today and the bottom hose is not heating up like normal and only one of the heater core hoses is warm. weird. Any pointers anyone?