Hi. I used to do lots of gym and running before DS (2.5), and got back into it soon after, doing a half marathon when he was 7 mths and BF. But still not as fit as before as I just don't have the same amount of time to devote to it, runs and classes are slotted in. Whereas before DS, would spin and if felt good could just stay and do another class or hit the treadmill, now I rush home. Running, I used to go to a club, where would be pushed to do speed work or hills, or go with my much faster DH, now just head out on my own for 30-60 mins steady.
Even when they're sleeping at night, we're not getting lie-ins, or much rest during the day!
If you're not doing so many of the high intensity classes anymore, may effect your running speed and stamina, it does for me.
Adding a spin class once or twice a week has increased my running pace.
So in my case, I don't think its BF, just having different priorities.
Maybe have your iron levels tested, as if you're a bit anaemic after pregnancy/birth could be a factor in tiredness, but I'm sure you'll be back to full fitness soon, its still early days. Its not just the 9 mths since having your DD, your fitness will have declined a bit during pregnancy too.
I felt my fitness had taken a huge hit and that I wasn't improving; did 2h30 for half marathon when DS 7mths, when before having him I'd had done 1h54. But a year later I did 2h06, and hoping to get closer to pre-preg time next month. Still BF, and doing nowhere near the amount of exercise I used to, but getting there back there slowly!