20 mins awake time after a feed sounds fairly normal to me. I think DD was about 12 weeks before she was having an hour of awake time total.

Keep an eye out for the tired signs, rather than going by the clock. Wait until you see a yawns, rubbing eyes, nestling into your shoulder and pop her into bed.

It may be that she is getting a bit overtired during the day and is having trouble settling herself back to sleep after a sleep cycle.