Feeling like roommates...?

So last night my SO kinda mentioned something about sometimes just feeling like my roommate.? We had a beautiful baby girl who just turned two months we haven't had sex yet (we tired lastnight but It was uncomfortable still) we've been fighting a lot & just kinda dissent. We've been together over five years & this isn't the first time one of us felt like just a "roommate" or that there's not a "spark". It's like a rollercoaster when we're good were great but then we have our down times too. There is a lot to it but I was just wondering does any other couples do through this? Or some girls feel this way? & does anyone have any advice on how to fix it?