I only come in one position

Yes, most women have a favourite position in which they find it easier to orgasm, which can even be the only position in which they can orgasm.  There are several reasons for this phenomenon: stimulation of a particularly sensitive area inside the vagina seems to be essential.  In some women, this is the G spot.

Other women can only reach orgasm with their legs squeezed tightly together, and some need their legs wide apart.  This depends on the tone and position of certain muscles, which can facilitate or block orgasm.

Sometimes, a certain position can be more mentally than physically stimulating.  It may mean that your bodies are touching in a certain way, or you may have an especially erotic view of yourself or your partner.  Mental stimulation plays a huge part in orgasm, another reason why you may find it easier in certain positions.