Water bed belly. Should I be worried?

This sounds very much like you have a seroma.  This fluid collection under the skin will need to be drained, either in the office, by a radiologist, or in the operating room.

From your description, it sounds as if your fluid collection is a seroma. These are best off treated sooner rather than later. This is typically done by one or more aspirations of the fluid. Contact your plastic surgeon now and let them know what is going on.

It sound to me that you have a sermon that need to be drained. Please contact you surgeon to have it treated as it can be managed in the office with a relatively short procedure.

Based on your description, I think you will be best off seeking follow-up with your plastic surgeon, earlier than already scheduled, if necessary.  Your description is consistent with  the development of a fluid accumulation called a seroma.  Treatment often involves aspiration (sometimes multiple). Best wishes.

When in doubt please contact your board certified plastic surgeon and scheudle a follow-up appointment, As surgeons we need to know if our patients have any concerns so that these can be addressed. 

It is normal to have swelling accumulate after having a tummy tuck and the drain is removed. Usually the excess fluid will absorb within time. It is important for you to wear your compression garment as recommended by your PS. If the fluid continues to accumulate or does not absorb you should contact your PS with concerns that you may be forming what is known as a seroma.