What Causes A Lump At The Base Of The Penis?

i have a small seed like lump(5 mm dia ) on the side of my penis. Its almost attached to the tube on the left side at the base of the penis. Its not painful but its hard. During an erection i have pain at the base of penis and also the two sides of the penis muscle. what could this be . please advise as my GP says its probably a cyst. But why the pain ( since 1 month). reply to YYYY@YYYY .

thanks doc.