Annoyed!

Hey guys! So, I am currently TTC. I'm almost 24 and I have a 9 month old currently. Before I had her, my cycle was VERY regular - I would get AF every 28 days exactly, so it was very easy to tell when I was pregnant the first time. I got my Mirena out about 3 months ago. Last month, I was sooo excited, thinking I was getting my BFP. I just ended up being about 10 days late. Here I am again, wishfully taking HPT after HPT at 5 days late.... Notta. Is that just common to be super irregular even 9 months after baby #1? Any info/advice is welcome!