My DD is 2 and on the pathway for an ASD diagnosis. She also repeats words over and over and over. The SLT discharged her because she knows alot of words and can follow very simple commands (when she wants to!). However, the paediatrician overseeing her assessment picked up on her repetitive use of language but also the tone she uses. It's always very emphatic and exaggerated. I didnt even know that was linked to autism until I looked it up.
I also thought my DD made great eye contact. Again the paediatrician pointed out the times where DD should've been making eye contact but just wouldn't. Now I know what to look for it's more obvious.
As for headbutting the floor, my DD does this. When she is having a tantrum she will find a hard surface to bang her head, when she is a good mood she will bang her head on soft surfaces. My 1yo copies her by banging her head on a hard surface but isn't suspected to have ASD. HV told me not to stop DD because she is seeking the sensory stimulation and although may cry because of it she wont do any actual damage.
Not all DC with ASD arm flap or toe walk. I have another DC with ASD and he has never arm flapped.
It is worrying though when you see things and just have a feeling. Have you spoken to your HV? Have there been other areas of concern flagged up?