I am 21y and I have a painless lump underneath the skin of my penis, on the right side beneath the head/glans. circumcised. I don't know exactly how long it's been there, a few weeks maybe. Could be related to masturbation? it's not visible but can be feel when I push the crown of penis. 
looks like this?
Hi Ricky, I need to check out the full range of possibilities of what can be going on for you to come up with an explanation: so, do you constrain the penis in any way, or engage in any BDSM practices? Do you practice natural penile enlargement?
If you do nothing like this then of course there is a possibility it is linked to masturbation, but, as I am getting at, this kind of thing is linked to exerting considerable pressure on the penis, and it is easier to harm the penis when it is erect rather than flaccid.
Penile cysts are quite common, for example, but these normally form a large garden pea type shape - but this looks more like a band shape across your penis, would that be right … ?
If this is right I think the only way you are going to find out whether this is harmless or not is to get it checked by a Doctor …