BBL on a petite woman: I am 4'9 and my normal weight is around 110lbs.

The question of the perfect weight as it relates to BBL is fast becoming one of the most commonly posted questions in the RealSelf community. While it is true BBL is a weight sensitive procedure (in the sense that you need fat to achieve your result), it is never a good idea to pursue short term changes for surgery.
While there is no absolute number (lbs.) in terms of weight for undergoing liposuction with autologous fat transfer, the optimal candidate has reached a healthy and stable weight (preferably with a BMI less than 30, which is associated with a lower incidence of complications).
Potential candidates should also have adequate fat reserves for transfer. This becomes especially important when one considers the anticipated rate of fat resorption and graft loss. I typically recommened patients aim for at least 500 cc transferred per cheek. This would amount to 1 liter of fat (not lipoaspirate) after processing. Candidates with inadequate fat reserves are likely to be disappointed with their ultimate volume once healing is complete. 
With regards to your specific question, limited fat reserves do not necessarily prohibit surgery but they do limit your results. You may want to consider implants in conjunction with limited fat transfer. Alternatively, fat transfer alone remains an option as long as you are willing to accept a more modest result.
As always, discuss your concerns with a board certified plastic surgeon (ABPS)

Thank you for your question. An in
person consultation with a board certified plastic surgeon is best for
individualized advice. At that time your body will be fully evaluated. A
surgical plan is tailored to specifically address your body. 

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.