We are happy to have you join us. Now, lets see what we can do to remedy the situation.
First of all, you need a black plastic cover on the ground below your house (I didn't see any mention of a basement). The plastic will make it much easier to work around under the house.
Second, you will most likely find a place where the Plumber cut a floor joist into for his pipe to pass through. The other joists around it can't keep bearing the extra weight, so it is sagging.
You will need 4 pieces of 4 X 6 about 16" long to lay on the ground (use treated lumber and it will be fine on the plastic), 2 pieces of 4 X 6 about 4' long for beams to go across 3 or more floor joists, placing one beam on each side of the cut; rent 4 jacks that you feel you can handle under the floor.
Place the 2 beams up against the floor joists (hang them with plumber's roll and roofing nails), place the 4 blocks under or near the ends of the beams, set the jacks and start raising each of them a little at a time (you will need someone inside to watch for the right height). When you get the right level and are satisfied, cut pieces of 4 x 4 or 4 X 6 for posts tightly fitted and remove the jacks.
Please let us know how it works out for you and maybe send some pictures of before and after.