Thinking I'm gay (or maybe bi) whilst in a hetero relationship?

I've been with my boyfriend for almost 2 years and love him very much as a person - however I've had no desire to have sex or see him naked etc. for the last year or so. In this time I've had a lot of sex dreams about girls and have even noticed myself having small crushes on girls.How do I know if I'm gay? These feelings have only arisen in me slowly over the last few years and in most stories I've heard, people have known their sexual preference from childhood. I've only ever dated boys and I've always been comfortable with this until now.Additionally, has anyone been in a similar situation of being in a hetero relationship whilst questioning their sexuality? What did you do?