What Causes Pain And Discomfort In Upper Right Abdomen?

Hi I am 26, female, 5'10", 147 lbs. I have never had any serious medical problems. For about the past month 1 1/2-2 months I have had a weird feeling in my upper, right abdomen. First, it felt like there was a knot there, or a lump, but there was nothing there when you would feel with your hands. This is the upper right side, just under the ribs. Randomly I will feel a dull ache or dull pain there. The other day I felt something like what it feels like when a baby kicks inside when you are pregnant in that same area (I am not pregnant, and this is upper right side just under the ribs). I feel like i have to exert extra effort when using the muscles in that region (i.e. vacuuming, lifting a child, etc) I guess it kind of feels weak. I don't think I have any other symptoms and was wondering if it could be some kind of muscle strain (if so, how long would that take to heal?) or if it is something I should go get checked out.