The red light flashes for only two reasons:
- The heater is too hot
- the Temp Sensor is bad
Last one first: if the sensor is bad, replace it. If it was working before you did the water change, it is most likely fine.
The heater can get hot for only one reason: water flow through the heater being too low. Several easy things to check regarding that: You said you 'rinsed' the filter. That does nothing for oil, and oil can block a filter which looks clean - you need to actually 'clean' the filter with a filter cleaner. For most folks, doing so once a year is fine, rinse the rest of the year. If you have lots of oil, or older filters, you may need to clean them more often.
If you get an air bubble in the circ pump, it may or may not start up by itself. A brand new pump will, but as they age it gets tougher, and I simply recommend you avoid the problem by never letting air get in there, or by blowing it out by putting the hose into the filter core as you refill every time. That primes it, and hopefully blows air out for you. You have two drains - only one is designed for water changes. The other is for draining the plumbing in case of freeze or other special situation, and using it almost always introduces air into the circ pump. If you feel you want to keep that drain clean and clear, wait until you have a full tub, add a shock treatment of chlorine, and open the cap for a minute then recap. Doing this once a year is usually fine.
HTH
