Have you had any labs done yet (maybe a bit early, but some do them at 3 months,)? One thought is a bit of anemia, either from iron or B12 levels being too low.
The other thought is diet - most of us are/were on a low carbohydrate diet by default (low calories, protein emphasis, not much room for other things...) and some take that to greater extremes by following one of the popular diets like Atkins, keto, paleo, etc. under the belief that they will improve weight loss results. When one is being particularly active, low carb diets can yield this kind of result, either overall lethargy, or running out of gas quickly. When on a low carb diet, our glycogen stores (short term energy reserves of carbohydrate) are being kept low, and it takes time to replace it with converting energy from fat - either stored or dietary - so we can wind up running short. It's like only filling your gas tank to half - it may not be a big deal if one is a stay at home sort making few errands, but if one is active and always out and about doing things, then one needs to keep filling up.
I never worked to keep carb intake particularly low (or high - I simply ignored it) and never had any particular overall energy issues after the first few weeks. One thing I did run into at about four months was running out of gas after about an hour in the pool (just lap swimming). My RD suggested trying a pre-workout snack/meal that was relatively high in complex carbohydrate, moderate in protein and low to moderate in fat. I played with that and settled on a small sandwich of whole grain bread, meat and cheese, and that did the trick - breaking through that one hour wall. Hypothetically, that provided a bit of a blood sugar rise at about the time that I was otherwise starting to run low (or it was topping up my glycogen supply so that I had enough to go farther.) Note that this made no difference on the days that I did a weight circuit, as I never had any energy problems then, even though those workouts ran upwards of 90 minutes. So different activities can create different demands, so that is something to play with.