Yes, abx can cause weight gain and water retention. Important to avoid dehydration, because that just makes the body retain even more water.
Keeping electrolytes in balance is also important - a high sodium to potassium ratio will definitely cause water retention/weight gain...so eat lots of fresh veggies!!!
Other causes of bloating - if you mean abdominal distention, then Lyme die-off itself is a big culprit of this, as is activity of the Lyme.
Candida yeast can also do this, as can bacterial dysbiosis - that is an imbalance in the type of bacteria normally present in the gut.
C-Diff may also do this, though I think that is more likely to cause diarrhea and not so much the distention.
If by bloating you mean fullness (which is how the doctors use this term), then that too can be caused by Candida, Lyme die-off, or Lyme activity itself.
Also, the high fiber, low carb diets recommended for Lyme patients can also lead to fullness (fiber slows down stomach emptying), and can also lead to abdominal distention because of the excess gas stirred up by the fiber (usually more of a problem for those who are new to the diet and not accustomed to the high fiber).
For recipe ideas that are Lyme-friendly (low carb, low sugar), check out
www.paleofood.com/