Hello and welcome to HCM,
Without being able to do a physical and examine your abdomen, it is hard for me to be able to tell you what is going on, but it's possible you are describing an
umbilical hernia.
Hernias can happen anywhere in the body that there is a weak area and the
umbilicus is one of them. But usually umbilical (abdominal) hernias do not continue to make your abdomen progressively larger over time - at least that's been my experience with patients who have had these.
Usually with an umbilical hernia, when you sit up from lying on your back, or strain, the herniated area becomes more prominent with a soft bulge near the navel.
If your doctor feels it isn't a hernia, an
ultrasound or CT may be helpful in providing more information.
Here is a link with some information about umbilical hernias. https://www.facs.org/~/media/files/education/patient%20ed/adultumbilical.ashx
There are other possible things on the list of what might be going on, but given that it came on abruptly after lifting something heavy, I think a hernia is possible.
You are doing the right thing to get this checked out by going to your doctor. If it is a hernia, it can get repaired. If it is not a hernia, it is time to look into what is going on.
I hope this information helps.