Two things happens during intercourse that do not happen during masturbation. Yout uterus will lift up inside of your body. This will cause the upper part of your vagina to 'tent' (forms a large open space). These are both natural and can be the cause for this feeling.
MRIs of couples having sex also showed that your bladder will quickly fill during sexual arousal and stimulation. All this coupled with the increase in blood flow, the swelling of your genitals and breasts can make you feel bloated.
There is no real way to 'fix' this as it is normal and natural. Stop concentrating on it and concentrate on the incredible feelings in your genitals and how to make it increase the whole time you are having sex. Also try to have the sexual tension in your muscles build the whole time.
Do not start to concentrate or fixate on any other feeling - only keeping the pleasure and tension building.