I had a lump a few months ago and like you I was due my period - but unlike you I never thought of that so went to the GP, she checked and asked me to come back after my period.
Lump was still there, so she referred me to the breast unit, the appointment was within two weeks of her referring me so can't complain. The doctor examined me and did a needle biopsy, which showed it was a cyst. He drained it completely.
They then referred me for a mammogram and I saw the doctor again a week after that, they confirmed no other bumps, lumps or worrying bits on the mammogram, but asked me to come back in 3 months.
At the 3 month appointment they did a further examination and advised me if I had any more lumps NOT to assume that it was another cyst but to see a doctor. For the next 12 months I can self refer to the unit without going via my GP, after that it needs a GP referral.
I was so impressed by it all, how thorough they were and also how kind they all were. The NHS really can be fantastic.
Go get it checked out, it is far better than worrying.