I Have Had A Discomfort In The Posterior Lower Area

I have had a discomfort in the posterior lower area of my rib cage only noticeable when I take a deep breath and especially if I am lying on my back. On a scale of 1 to 5 it is less than a 2 when present. I have had it for 3 weeks now and while it is no worse, it hasn't subsided. It feels like something is pressing against the rib cage. I don't want to rush to the doctor if it's just a pulled muscle but it seems like quite a while for a strained muscle.