sounds to me like it is your heater. it probably malfunctioned., mine did the smae thing but pushed my temps to 87 (but i was treating for ich... how convenient )
some heaters just go crazy and stop working or get off. its hard to know 100% what it is. your the only one who can test a few of the ideas and see what it is. has anything changed? more lighting on more room temp ext
the only thing is we leave the ac off during the day so it get in the mid eighties in the house. other then that nothing else. the heater light is not on either