15 wks pregnant and feeling like I have a huge rock weighing down my tummy?

It is so weird...I have a 22 mth old and it never felt like this with him. It feels sooo heavy and I know the baby is very low. I feel like the baby is pushing on me..sometimes I feel the urge to push. It is so odd and very uncomfortable..like I am carrying this heavy weight and my back is killing me because of it. ANyone else experience a feeling like this.