One thing to keep in mind that is all-too-often overlooked is that it took your body nine months to get to where it is now. Don't expect it to bounce right back to where it was overnight. Knowing that you are athletic and breastfeeding is a positive thing though. The breastfeeding can (not always though) help you to lose some of the 'baby weight' you gained during your pregnancy. And, of course, being physically active will help too.
If the capris being a size bigger works for you, then I would suggest to continue in that size. If the belly band is uncomfortable, then maybe one size larger might be a more comfy fit. I would expect to wear that size for close to a year (considering it took so long to get your body where it is now and will take time to get back). Now, it's likely that your body never will be exactly as it was before. You carried a baby and that changes things, lol. But, by taking care of yourself and getting exercise will only help in getting you back to some level of normalcy.
Congratulations and God bless,
A.