help!

My LO turned 5 months today. I tried giving him cereal at 4 months but he stopped sleeping and was super fussy so we decided to wait til 5 months. So after his morning nap today I gave him like maybe a teaspoon full of oatmeal baby cereal in an ounce of breast milk. He ate all of it and did really well. About 3 hours later he spit up A LOT. Like more than he ever has before. He also kept gagging afterwards and spitting up little bits more. About an hour and a half after that he nursed just from one side. I sat him up and was burping him and he projectile vomited a lot even more than the first time. Like prbly all of what he had just drank. So my concern is, when do I try nursing him again?? What if he keeps vomitting?? Idk I guess I just don't know what to do and I feel really bad. 😞 any thoughts or advice would be appreciated!