WHat to do??? I get an Erec-tn out of nowhere...!?

Im 21 and I have had no ual encounter. WHen I talk with female friends, I get a "b-ner" , Its very umbarring bcus if forces me to be sitted down until it relaxes.. It happens even with friends that may not look very attractice to me and our conversation is very conserved- not about our ual live of anything like that.Is that normal??