I'll try to keep the background as short as possible.
The first few months at my current job were not easy. My direct supervisor was awful, the training was rubbish and I basically used to dread going to work. The good work that I was doing was being overlooked and I was being blamed for things I hadn't had anything to do with. Safe to say it was one of the most horrible phases of my life.
And then something amazing happened. One of my superiors from the head office (most of my tasks in the job involved working for him remotely) told me he loved my work. He went out of his way to call my direct supervisors and tell them how well I was doing- note that this was his call, I didn't ask him to do it. Given his position within the company, people suddenly sat up to take notice of my work and the bullying started to gradually stop. I sometimes feel that I may not have been able to stay in this job had he not stepped in and clarified the quality of the work that I was doing. Needless to say, I am incredibly grateful to him.
Our work relationship has been amazing. He's notoriously hard to please, but it's been such a good experience working with him. He's been happy to share his knowledge, he's encouraged my work and we also reached a comfortable stage were would chat about things outside of work.
Unfortunately, I was moved to a new product and I no longer get to work with him much. He did mention to me once that he is not very happy with the person who is handling his work instead of me now. Given my relative experience in the earlier product, i still step in from time to time to help out. Initially, he was responsive and even happy on the occasions where I would step back in. However, there have been two recent occasions where much to my confusion, he has ignored my communication when I stepped back in. 
The new person forgot to let him know something important and I noticed it in the system. So I just sent a message to say- hi, here is what has happened. Noticed it and thought to let you know.
I have sent similar messages in the past and he has responded and has been appreciative and thanked me for still keeping a check. But on these past two occasions he completely ignored me :( No response. Absolutely nothing.
I had also sent him a message to ask a work related question he might have had the answer to. He completely ignored that as well. I found this strange and out of character because he has always been so happy to share his knowledge in the past.
Am I overreacting?? I feel terrible that he has ignored me because even though I am not on that product anymore, I still value this professional relationship a great deal. Besides I might at some point come back to the earlier product, so keeping my contacts and relationships alive is also very important for me.
I am struggling to see what I might have done unknowingly perhaps to upset him or annoy him to the point where he would behave so out of character and ignore me
I know it sounds odd to take this so seriously, but his support gave me a sense of security within a work environment that can be quite hostile at times. I am not in a position to switch jobs at the moment, so losing that support is making me feel rather lost.