Happy in the day, Sick in the night

My poor baby, it seems like he gets off an antibiodic and then he catches something else. He just got off of one on Monday and now has who knows what. Yesterday he started not feeling good around 6:30 at night and was running a fever of 102. We gave him tylenol it dropped his temperature and around 3 in the morning Timothy was able to put Weston in his own bed and come to bed as well. They have been sleeping in the chair. This morning Weston was doing good, I mean he was a little tired and not playing as much as usual but I figured well he was sick last night he wont be up to par just yet. He drank some from his sippy cup and even took some milk from the bottle. So we put off going to see his doctor because I thought he was feeling better. Then around 5 ish, tonight he started throwing up and running a temp of 101. So gave him tylenol again and he started to feel a little better around 7 and then all of a sudden when we started his last feed, he starts throwing up again, his heart rate went into the 200's and he dropped his oxygen only a little down to 95. So we stopped his feed and undressed his trying to get his temp down which now it was up to 103. Right now he is sleeping on Daddy's chest and stating good, 98 and heart rate 119. We are going to the doctor even if he is jumping off the walls. I just don't understand how he could be feeling good this morning and then this evening be so different. I worry about him so much. Just hope we can figure out what is going on and make him feel better.