I thought it was a myth but have woken this morning empty!!

I have exclusively BF my DS for over 12 weeks now. For the past few days in the evening he is taking ages to feed. I express at 9.30pm and always get about 150ml and give him a bottle of Ebm but last night only got 70ml and had to top him up and he had a total of 230ml. He has slept through the night since he was 5 weeks. When I wake at 7 I'm usually very full and leaking but this morning they are empty. I expressed to see how much I got get, I only got 20mls!

Is this over for me? I was going to wean in the next few months anyway but I'm really disappointed as it's not on my terms. Why has this happened? How can it happen? Has it happened to anyone else?

I'm using my phone so hopefully this post is ok