How a sewing machine works

Sounds like a bent needle or improper tension on the upper thread. Change to a fresh needle and see if that helps.

I've actually found out that most of the time I just need to re-thread the upper thread! Idk, as a beginner this is all strange and new to me, that just yesterday the machine was sewing fine and then the next day, without changing anything and working with the same fabric, it behaves like an arse. Thank you for the tip though, I knew about the tension part but didn't realize it could also be a bent needle!

I feel your pain. Trying to figure out why the darn thing isn't working right can be a challenge. Apparently there are things like thread that's too thick/skinny, fabric that's too fuzzy/slippery/elastic/thin/thick, dust buildup behind the lower thread spool and the wrong stitch length. But when it works well it works well.