I can feel a lump?

L

the last couple of mornings lying flat on my back I can feel a slightly hard lump above my pubic bone. I'm only just 10 weeks and I thought you couldnt feel the uterus outside the pelvis until at least 12 weeks. I'm terrified it's twins - I'm sure of my last period date and I've already adjusted my dates because I know I ovulated early. Anyone else? Thoughts?