i have had this happen, not in relation to EMDR though. but yes. it is likely a protective mechanism and/or part that is blocking things in order to protect you. it could mean you are not ready for it and it was unsafe even if it was not related to a memory in your consciousness.

i have had this when things are too much whether it's too much trying to surface or saying too much in general to someone (even if i have said it before to them).

all you can really do is accept it right now and see how things go moving on. it might not want you to work on that particular thing right now. pushing it could do more harm than good too. that is partly why my therapist won't do EMDR with me because of not knowing the parts enough or having them all on board and the potential for flooding.