My 6 month old has diarrhea...bad?

My baby boy has been eating rice cereal with juice for a while now. And he's never had a problem. In fact, I hoped it would help him go to the bathroom easier, he's always had big hard turds instead of the looser stools. I breastfed for about 4 weeks and then had to formula feed. But even with the juice it was only sort of helping. But after he had 2 boxes of rice cereal gone we decided to try some baby oatmeal for him with some juice and he LOVES it! It's his favorite even more than the rice which he liked a lot. And now, his poop is yellow and runny and he's going anywhere from 2-9 times a day. He is getting 3 teeth and already has 2 teeth in and with the first 2 he just spit up (he was never a spit up baby until he started teething). He never got a fever or anything. But this time around he's fussy and I have to give him Tylenol at night to help him sleep. But I don't know why he has diarrhea. I've stopped giving him juice and began mixing his oatmeal with warm formula which he doesn't like as much and filling his sippy cup with water which he enjoys chomping on and spitting the water out (silly little man). If his diarrhea doesn't stop after his oatmeal is gone (he's only got like one more serving left, at most 2) then I'm going to take him to the doctor, don't worry I'm not neglecting him or anything. But the other thing that is confusing, is that he doesn't fuss about a tummy ache or anything. He hasn't had any Tylenol for his teeth in over 2 weeks and the same with juice. So I think it's the oatmeal but I'm not 100%...We're going to get some pedialite for him to see if that helps as well. Do you think it's just the oatmeal?