Breast feeding diet

Yes, BF has been proven to help you lose weight quicker after giving birth, it burns a surprisingly large number of calories... As well as having many long term health benefits for both you and your daughter! Keep it up!

When you start exercising, make sure you drink LOTS of water! Which you may already be doing now you're BF, but when you add some exercise back into your life, even if it's only gentle, make sure to drink even more water than you usually would if only BF. Depending on where you live e.g.: what country.. If you're in a country where you're heading into the Summer/Warmer months that's also something to take into account.

If you feel your supply has decreased after a few days of exercise, or your daughter shows signs, maybe she's a bit fussy or shorter time between feeds... If you have been drinking plenty of water, attach your little one to the breast (maybe 3-5 mins each side) put her to the breast for a quick feed either just prior to or after you exercise (depending when you last fed her). This won't affect her usual feeding routine as in those couple minutes she'd just be getting the 'foremilk', its very watery milk, not very filling... It'll be of benefit for both of you especially if the weather is getting warmer where you live.. It'll keep her hydrated and your supply levels up.
Remember... She demands, you supply! The more she feeds, even those couple minutes, the more your body produces to keep up with.what she's demanding.
If you're still worried after trying all that, you can get a 'herbal supplement' from a health food store that is good and assists in milk supply/production (as long as you're still BF) the 'herbal supplement' is called "Fenugreek" there is only limited research about how effective this supplement is regarding milk production etc, but it is 100% safe to use during breastfeeding (as long as you don't have an allergy to it) and despite the limited research, people do say it works.
Failing that, and if your daughter is starting to lose weight, go see your Doctor especially if you feel it's due to your supply.. There are common prescription medication used for nausea and vomiting, called anti-emetics that area commonly used to increases breast milk supply.

Sorry it's so long, hope this has been of some help and not scared you or made you think you need to "put off" exercising..I've given you the above info from my nursing/midwifery experience.
From my personal experience, i went out and played five, 6 game, sets of state league tennis matches, both doubles and singles when my son was 5 weeks old, I had to stop, ask for 20 min break during my singles match between the 2nd & 3rd set to feed him, otherwise I would have had to forfeit my 3rd set... It was 37 degrees Celsius (plus approx 10 degrees on a tennis court) in the hot Australian summer, the exercise, temperature etc that day didn't adversely affect my supply... I fed my son till he was nearly 3 yrs old.
Good luck to you both though. I'm sure everything will be just fine. X