Problem when sculpting- progress dissappears

I’m new to Blender. I’ve used it for about 2 weeks now. All’s fine till today.

So I’m making this character model for a game and I’m up to sculpting the guy. He is not one single piece however. He’s made of multiple objects and I constantly switch between them when sculpting. Each piece I work on has multires modifier (with different amount of subdivisions, but most of them are on 4, which is what my PC can handle before major lag anyway). I do some sculpting on the head which misaligns with his scarf, so I switch to that object and do some sculpting there and then when I wanted to go back to the head, i noticed that it has lost all sculpting progress so far. This is the second time it happens! Sure it might be a random bug once, but twice? No way. And the first time when I lost progress I’ve actually saved the blend file and come to the program hours later, to find progress lost when opening the file. This time however it happened mid-session! Haven’t closed the program, just working on a different part of the model when I noticed that the previous part was “reset”. I think it may be something to do with memory usage, but some friend said he’s working with 1.4 Gb just fine, while my blender file is only at 130 Mb. So what the problem here?
And if it tied to memory, can I change how much memory it uses from user setting or something?