My 3 1/2 year old has strep throat & is now on amoxicillin (day #2). Now I have a bad headache, fever & a sore throat. Is this strep throat now?

Could be. The family transfer rate runs ~1/4 at this time of the yr. Less likely >24hr of meds but you both may have been exposed to the same source or he could pass it to you before rx. A quick strep test with a culture if that is negative ( few false neg tests occur) will let u know.Great handwashing & lack of oral contact keeps transfer rate down, a difficult thing in families.
Possibly. You do have positive contact...Possibly from kissing your child many times a day as i did when mine were little. Call your doc to have a rapid strep test done in the office... You will have an answer in about 5 minutes. In the meantime, Motrin or excedrine may help with the headaches. Good luck.