Hi there
In my experience, there are ups and downs. I also had periods when LG did not gain. I exclusively breastfed for 6 months, I still breastfeed, but LG gets one bottle of formula when I am at work.
Did your LG not gain for a whole month, or just a few weeks? If it is just a week or two, then it could be because she is sick. The Kellymom website says that babies who are sick (with even a minor illness) often slow weight gain or even lose weight. This is especially true if they have a stuffy nose
According to the Kellymom website the average daily intake (24 hour period) of an exclusively expressed breastfed baby is about 750 mL, and can range between 570-900 mL per day. How many times per day does your baby feed? If she drinks 7 bottles of 100ml, then that could be enough for her. You should also remember to count night feeds

Some babies reverse-cycle, ie they drink just enough from the bottle to take the edge off and then drink a lot at night (straight from the source

) to make up for the day. If she has 6-8 wet/dirty nappies per day, you are most likely on the right track.
This article gives you a list of reasons why your baby may not be gaining weight and what you can do to improve weight gain:
kellymom.com/health/growth/weight-gain_increase/
If your baby is fussy after feeds or you still feel worried, rather be safe than sorry and go see your Pead.
Well done mama for exclusively breastfeeding, hang in there