Yep, it's not really likely that bot fly larva are causing weight loss. A fecal exam would be a very good idea, though getting one done right after using ivermectin is likely to just give you a negative result because of the dewormer and not tell you what kind of parasites your horse is picking up from the pasture.
How old is this horse and what breed? Is the hay you are feeding really good quality? And you say it's free-choice--available 24/7?
Besides the last deworming with ivermectin, what is her deworming history like? What kind of pasture situation is there---other horses, feces picked up, etc? Being that you are in Texas, you are seeing the time of year with the highest parasite reinfection rates. Depending on your pasture maintenance, that could be really high or it could be pretty low for your specific situation, but you need to assess the risk factors and get some basic info on the kind of parasite loads your horses typically carry by having fecal egg counts done on them at least 9-10 weeks after dosing with ivermectin.
Licensed Veterinary Technician