pink stain and fever up to 38°c

 4 month old is having a fever up to 38°c and it's been on and off for 2 nights. Sometimes his temperature is normal sometimes not. Especially @3pm onward. And just in, I saw a pink stain during his diaper change one time. Is there anyone has the same case like I do and did anyone got well informed? Anyone who have the same case and already had pedia response? My pedia just told me to observe but right now with this pink stain I'm starting to freak out since last month he have the same case and diagnosed with ecoli bacteria. I'm confused where did he got the bacteria since I am changing his diaper round the clock every 3-4hours. I'm more on breastfeeding him than giving him formula if I ran out of milk. I'm getting paranoid about it.