If my girlfreind honestly says she loves me shal i belive her?

because i think she stil likes the boi hu she used to b4 we got togeher , she flirts wiv him sumtymz , i get relli jelouse , buh she says she relli loves me , im the one, should i belive her , because i love her so much nd hate tht boi , ?? what shal i do ??