First, determine if the sensor might be faulty by putting it in an enclosed box and see if it goes off. If it passes that test, then I would try repositioning the sensor elsewhere in the room, maybe lower on the wall with less exposure to the windows. You could also add another motion sensor in the room and make it so that both sensors have to go off before sending you a notification.
UPDATE: I’m don’t know what version hub you have or brand of motion sensor but with the recent hub update, there have been numerous reports of false motion sensors going off
For a more complex solution to false alarms, check out this thread