9 month baby waking up crying

My 9 month old baby sometimes wakes up in the middle of the night crying a lot ,it takes some time to console him back to sleep.checked diaper and no poop or pee.we cosleep and m right next to him..i just had his first tooth erupt mid way.what could be the cause? Is it the tooth? Or sleep regression? I started feeding him boiled eggs and veggie rice.mayb he allergic to eggs? Cant figure out as its first time he is doing it.how to know the cause?