Maybe, therein lies your problem. From what I’ve heard…if you’re just starting out (and you only started out 2 weeks ago)…you can keep the exercise 15-30 minutes…and then gradually increase the time. Trying to do too much can burn you out…and even demotivate you.
It could even be the combination of your exercises. You say you do 15 minutes of cardio…and usually cardio exercises (such as running) will work out all part of your body…so maybe there’s no need for you to do the upper/lower body workout after your cardio. You can just go to weights after cardio. You can even reserve weights until before going to sleep. It is recommended to do weights before bed time…because while you sleep…your body repairs your muscles…and this “repairing” burns extra calories during your sleep.
Also (I know you’re not sick)…but you’re not supposed to work out when you’re ill…as that will make you ill-er. Your immune system is using all its energy to fight the illness…and has little to spare for rigorous activity.