Yep, it's really common @Renatha The dread seroma. They can be a pain in the proverbial, but it's just another 'thing' you need to manage. Getting the fluid drained is a bit icky but it's a simple enough procedure.
Get in contact with you surgeon, your breast care nurse (or whom ever warned you about it in the first place):and they will organise to get it drained if they think it is necessary.
Some breast care nurses will do it or you may have to go into medical imaging and someone will stick an ultrasound guided needle in and aspirate the fluid. Sometimes that only needs to be done once, but seroma can be persistent so don't stress too much if it needs to be done a few times. Marg xxx