Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
It's possible that, once you freed up the space, it filled up again with cached this-and-that... e.g. a backlog of text messages (if you receive a lot of them) or other similar received data.
But what with other apps closing spontaneously, I would say that either
A. your software is corrupted... memory management not working right, or other things, or else
B. your phone may have been taken over, or hacked.
If it were an Android phone, I would suggest that you back up all data from it, then do a factory reset. As it is an iPhone, I need to ask first: what version of iPhone is it, how old is it, and has the iOS version on it been updated lately?
It may either be getting old - in hardware, or in software if the iOS has not been updated - or else other things may be wrong. Let me know, and I will advise you further.