Was it the baby I was feeling? Help

So this is all new to me. I've had past pregnancies but never made it past 13 weeks. I'm currently 17w1d and when I lie down sometimes tends to feel very hard on my lower abdomen usually above the pubic line and hurts when I push a little. This morning it was hard again only on the right side and a little squishy on the left. Then my right side was soft again and in the center under my belly button was hard. I used the Doppler and in that area was the hb. When i feel that hard lump is it the baby or the uterus? I've never understood how to find the uterus still but I know it's definitely moving up because the stretching pains are unreal and my stomach has gotten bigger and harder. I'm just curious that it's my baby I feel when I tend to feel hardness in one area more than the other. I know some have a pronounced lump but I don't where the hardness is.