Sleep regression

Kr

How did you know your baby was going through 4 month sleep regression? My little girl is 3 1/2 months and I think she is experiencing it already. Her naps are few and far between and fights her sleep when she does nap. When she finally falls asleep she’s awake again in 15-20 mins. Also waking up during the night when she’s slept from about 11:30-12 until 6:45-7 am since she was 3 weeks old.