Why is my girlfriend being so difficult?

right, me and my girlfriend hae been going out for about 3 months, everything was going good until about 3 weeks ago when she just suddenly changed, she started flirting more with other blokes and basically acting like i was a complete inconvienience, but amazing when we are alone she reverts back to her previous self and becomes feisty and full of energy, whats wrong , is she trying to make me jealous, i dont know