--line something in your sacral spine, where the nerve roots that give rise to the pudendal and penile nerves originate (at the S2/S3/S4 levels), may move when you do, and compress/irritate the nerves and produce symptoms.

Or, you may have compression further down the pubis--one site that is prone to this is Alcock's canal, which the pudendal nerve passes through (between the ischial spine and the bones you sit on) on the way to splitting into **** and penile branches.

Do you do a lot of sitting with bad posture? A lot of cycling? There is an entity known informally as "bicycle seat neuropathy" that involves pudendal nerve compression:

http://emedicine.medscape.com/article/91896-overview

http://www.ncbi.nlm.nih.gov/pubmed/11531032

And, have you had imaging studies of the pelvis/pudendal area?