Small (smaller than pea) smooth, round breast lump upper left breast. tender to touch. had appointment with nurse who said she wasnt too worried and that i should wait and see if it goes away after 3 weeks. im 25 and scared it is breast cancer?

Breast Cancer is very rare in women under 30 years of age. So it is unlikely to be the case with you especially because the size the lump you have felt is too small to take it seriously. I would suggest that you watch this lump for some months and check its size once per month. Unless it grows in size to 1cm, it is not likely to be breast cancer. Benign lumps like a fibroadenoma is more likely.

Additionally, breast cancer lumps are generally not tender as yours is.