Wow, what a stunner! Frame overo has to be my all-time favorite pinto pattern.
Momma really needs some weight on her or she will only get dragged down with the foal nursing because nutrients will go to the baby first.
I don't know how others will feel about alfalfa, but I had my mare on over 1/2 her hay as alfalfa when she was pregnant/nursing.
Purina Ultium Growth as someone else suggested is an excellent feed as well. I kept my mare/foal on it for about the first 6 months and then after that decided it was rather expensive to keep feeding. But it got my foal off to a great start.
I would worry more about momma's calories right now then her colicking. Any horse can colic, it's a chance we all have to take owning horses. Just make the feed changes gradually but get some more calories into her.
Alfalfa is great because it's high in protein and calcium, which is important for milk production and baby's growth. Oil is just calories, but does nothing for actual nutrients such as calcium, protein, vitamins/minerals, etc. I personally would want to feed calories that have more nutrition to a lactating mare.
Just my 2 cents on the subject, take it for what you will. Congratulations on a gorgeous foal!