Feel really down about stuff - down to relationship?

Hi all, I'm feeling really down about most things in my life at the moment, and I feel it's coming from my relationship. Sounds strange, but I have a good relationship with my girlfriend although recently it's got to the point where we bicker all the time, about nothing in particular. The rest of the time we couldnt be better, happier than most relationships! We spend loads of time together and I admit I give loads of attention and put in most of the effort... But for some reason I'm really depressed with the whole situation and not sure what to do about it... I dont wanna leave her cos I wanna work at it and think we could sort the bickering out (only 4 months in...) but at the same time should I keep trying at something that's getting me down so much?