Does its natural to cat lose a little bit of fur?

Hi. My cat have been pregnant 3 times. The first time ( before she got pregnant) there was a white cat attacking her and she lost a bunch of fur. Now she is fine. But know, since the kittens ( from 3rd birth) left. she is leaving at every corner hairballs ( not from her mouth). I saw her neck area, there is an area without hair ( a little area) and i'm getting scared. Why she is losing like 3 hours a little bit of fur. Please her. I'm going to spay her after a few days. But please help me. She hasn't been attacked like for 1 year ago. So she is safe and healthy. But I don't know now whats the problem.