Newbie here, and question about let down while pumping

thblock

I am a little new here.   I have a 4 month old DS who is EBF.  I have been back to work for about 6 weeks and pumping.  Today was so strange, this morning I pumped before I left for work where I usually get 3-4 oz it was like I couldn't get a letdown. I barley got 1 oz! Then it happened again at work where I usually I pump 5-7 oz during my first pumping session and I couldn't even get more than 1.  I don't think its a supply thing because I hand expressed a lot afterwards and my breasts felt full. I then came home and nursed because I was so worried about it and all seems fine, DS ate and seemed satisfied. Has this ever happened before to anyone?  I'm freaking out a bit.  I definitely wanted to EBF longer than 4 months and now I am worried about my work pumping sessions and supply, etc.  Any ideas?