First time TTC! Help!

Megan
Hello Ladies! My husband and I have been trying for our first baby for about 6 months, and each month my period would promptly start on time. My last period was from Oct 12-16 and I am now 13 days late. No pregnancy symptoms, just a lot of lower abdominal discomfort, and it feels hard behind/below my bellybutton. Took an HPT yesterday and it was negative (middle of the day). Have any of you ladies experienced anything like this? Help, encouragement, and wisdom are much needed! Thanks!!