Hi and welcome to the site.
There are a couple of things that I think of. First with a soft lump, there may be a weakness in the muscles and ligaments of the abdominal wall allowing some tissue from underneath to push through. This is called a hernia
. A hernia would more likely happen with heavy lifting or straining. If a hernia, the lump would be soft and be able to be pushed back into the abdomen.
The other possibility is a strain of the round ligament. This is a strong rope-like attachment of the uterus
to the side of the abdomen. It can get strained and sore with lifting too. And during pregnancy
, the ligament stretches as the uterus grows large. This can cause aching on the low sides.
Applying a heating pad
and avoiding straining would be helpful for either condition.
I hope this information has been helpful.