Why do i get this feeling?

its not the first time, it happens rarely but it happened before, i start dating a girl whose nice, and funny and overall great. and a few weeks or days into the relationship i feel as if the flame, the spark begins to die out.. and i know the other person feels it too i cant explain it but my heart tells me exactly how they feel. i know exactly how and why the fell that way. its extremely rare i asked people and they say this doesnt happen to them.. and i doubt im the only one.. i get this feeling like.. really weird feeling as if they are hiding something as if im loosing them because they dont feel the same.. the reason i ask this is because i just met this girl shes great. shes pretty shes just amazing<3.. i left my 2 year on and off relationship for her.. because my x cheated on me 5 times. that i know of. and lied to me constantly. so i left her for good i kinda feel bad but im happy with this new girl... so can anyone answer me. why do i get this feeling? its like the 3rd time this has happened to me. and how can i fix it. because when i start to get this feeling it doesnt go away until i loose her.. please help i dont want to loose her.