Breast pain when running

So I have to do running for sports day at my secondary school but I find when I run I have breast pain or even going down the stairs quickly and I'm only a 34 D but I have to hold them but I can't be running in front of the whole school holding them. I know there's a sports bras but they can be really expensive if you want them to actually work and I don't have much time to try them out. Are there other things I can try? Thank you guys, and I've had this pain since my breasts got around this size but it's not been as painful- and I've missed two periods almost three so I really don't know