Baby some times sleeping too much. not responding

Patient: My baby is 2.5 months old. She normally sleeps well. last 3 weeks (mainly after having vacinations) we observed some times she is sleeping 4 hours at a stretch. but more importantly if we try to awake her from sleep for feeding she is not wake up. we tried to knock him below her tow, rub her back, wash her face with wet tissue. but she is not woke up. she respond, move or stretch her legs. she woke up almost 1 hr we try. this is not a regular incidence. it happened may be once in 1 or 2 weeks.Normally she gets woke up in very littly touch or sound. she cry very less. her feeding and other routine, urine normal. but only this incidence we found unusual. please advise is it normal that SOME TIMES BABY CAN GO INTO VERY DEEP SLEEP?

Symptoms: Sleeping long time