Why is my horse off her feed?

I have a 3yr. old filly who is a little underweight and thus being fed Ultium. I put her in her stall, came back an hour later, to find that she had only eaten 1/2 her grain. I tried giving her a bite of another grain and she took and then spit it out. I watched her put her head back in her bucket and push her grain around but not take any. She is not sweating or does she have rapid breathing, no other colic signs. Poop is normal. She had a lot of pasture today, do horses ever just get full? Should I look at her teeth? She has colicked before so I'm really nervous about her. She also seems a bit lazy today...Please help me.