I just wanted to give some feedback here that may help others, after I experienced a scary situation myself and was panicking, searching for any information on the subject.
My son had his 6 month shots and had a fever (101.5), so I gave him Tylenol at the recommended dose before putting him to bed that night (acetaminophen 2.5 ml). Several hours later he was cool to the touch and I took his temperature again, both with a digital thermometer and rectal. He was at 94 degrees F.
This was terrifying, as technically it is in the hypothermic range. I checked the temp again to verify, then immediately stripped us both down, put a wool hat on him, then put him on my chest and wrapped us in blankets in order to bring up his body temperature. It took 3 hours of this insulated skin-to-skin to get him back to 98 degrees. I spoke with my doctor on call during this time to consult and they said as long as he was not exhibiting other symptoms (blue skin, shaking, disorientation, etc), this was the right thing to do.
The next morning he was acting normal again and his temp was at 99.5. We took him to the doctor and spoke at length about how this could have happened. I had done extensive research online and could not find any articles about vaccination causing low body temperature (as immunological responses typically raise body temp). The doctor looked into it further and we finally found an explanation:
---acetaminophen has an antipyretic action (lowers temp) and sometimes this can go too far in infants, bringing it to the hypothermic range. There are several European studies about this effect that you may find useful and reassuring, as I did! I hope this helps someone who may have this same experience and is similarly terrified.
Transient major hypothermia associated with acetaminophen: A pediatric case report and literature review: pubmed.ncbi.nlm.nih.gov/31331796/
Hypothermia: an unusual side effect of paracetamol
pubmed.ncbi.nlm.nih.gov/2652877/