If I were you, I would ditch the formula completely and contact a Lactation Consultant about bringing in your milk again. More babies have more problems digestively with formula...you wouldnt believe the things I see. It just isnt made for them to easily tolerate. Many Moms have to switch every few weeks to yet another new formula, and still end up having to medicate their babies because it just cant be digested and gives them a host of problems. It wont be difficult at all for you to begin your relactation journey,(you're only 3 weeks PP and odds are your milk is still there...you'd just have to build it up) so contact someone in that field if you are interested. Its the best for your little one, the best for you and will rid you of your issues. You can contact a local LC, a La Leche League Leader, or a Peer Counselor if you are a WIC Mom. You can contact me for more info. Good Luck!
*EDITED* You didnt mention you were nursing! You are amazing to still be able to offer even the small amount you do. ANY breastmilk is better than formula, so keep up the good work! May I ask what sort of conditions you have that affect your supply? I ask because I am a newly trained Breastfeeding Counselor (just graduated in September) so I like hearing different stories...more experience under my belt. Who knows, the info may help a woman in the future that I may work with. Either way, I dont know what measures you have taken to increse your supply or what you have been told. Mind sharing your story? You can e-mail me at ____@____.com. I'd love to hear anything you are willing to share! I'm glad the sitting up has helped her some! Thanks!