Most of the slow release ferts are temperature dependent, with higher levels released at higher temps. I would suggest overwatering as a more likely culprit. Don't water again until the surface few mm are dry. You'll start to get a feel for watering requirements by lifting the pot and feeling the weight. As it dries out it will get lighter. Cotyledons are more like an initial energy source for the plant than anything else. These should naturally wither and die as the plant starts developing true leaves and they start producing food for the plant through photosynthesis. Don't overdo it on nutrients, 1/4 strength of something like miracle grow is more than enough, and even then in soil you will have plenty of nutrition for a few weeks at least. Give it time, don't water until you really need to, and get a soil pH test kit soon. Good luck and I hope that helps somewhat. Btw most nutrient deficiencies are caused by pH rather than actual deficiency of the nutrient(s) in question. Overwatering can show as a deficiency like symptom too, as the plant has often lost a substantial par of the root system responsible for providing nutrient for itself. Good luck and don't worry too much, you'll get he hang of it soon enough. My best advice is patience and don't overanalyze such issues.