May have strained or have a pinched nerve along the neck area. The PBN (Press Behind Neck) is not for everyone, as the BP is not for everyone. Main problems with the PBN is that most people do not have enought flexability in the shoulder girdle at first, lower the bar too low, grip too wide or rush the weight on the bar too soon. I would guess that the main cause of trouble with the PBN would be improper warm-ups & stretches (for flexability) before doing the exercise. No matter, if that exercise keeps giving you trouble than try other delt movements.
If you seek a doctor attention he/she will probably either advise heat or cold packs, or both. Rest will also be suggested. Anti inflammation drugs, if the pain is severe, may also be advised. If it's really serious than a shot will be given. If it's an average, not too serious injury than the discomfort should go away with-in a week, if rest is given the area. Might also consider deep tissue massages, from time to time, after the injury has healed.
The PBN can be a prime muscle mass shoulder builder and has been for a lot of guy's over the years. 200-250 is usually the weight used for most experienced guy's, without any problems. But if it's not for you, than it's not for you. Lot of guy's just can't do regular mailiary press either without some troubles developing. . Good Luck.