Weight gain during pregnancy

I started my pregnancy at about 108 pounds, so I expected to gain weight on the higher end of the expected weight gain scales. During my first trimester I gained weight pretty quickly...a little over ten pounds. I stagnated for a while and stayed around 119 for several weeks until I suddenly gained a bunch of weight again. I've been about 130ish since Christmas time and haven't really seen the scale move since then besides daily fluctuation. I'm in my third trimester now (about 28 weeks) and everywhere I see says I should be gaining about a pound a week. Should I be concerned that I haven't really gained and kept on weight in the past four weeks? Or since I tend to gain weight in chunks at once, is it just my body working through the extra fat instead of gaining steadily?