I have this swollen lump right under the head of my penis and it feels like there is a hard vein inside the lump... It seems to get bigger when I masturbate or have sex.. It does not hurt or anything... This morning I woke up after having sex and it was a lot bigger and my lower back was really hurting me.. Don't know if that's related but it has shrank down smaller again since I had sex... Im so lost on this and it really freaks me out.

I suggest you visit your doctor for a visual diagnosis.

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