Was in a similar situation in reverse went to my doctor because I had phlegm on my chest approx 12 days before my flight to Thailand of course tested positive sh*t anyway was given amoxicillin to help clear phlegm which may be why others are using it of course with Thai pass .hotel booking for test and go .flights rising in prices etc etc I decided to chance my luck and take pcr test 1 day before due to fly (spit in tube) and result came back negative.
Flew to Thailand late Sunday arriving Monday test at hotel result positive got blood test at my expense 1200 baht plus got my doctor to send mail stating I had tested positive but had done my quarantine whatever the reason last night got out with warning to stay away from bars which I will do.
Since I thought I was going to be quarantined again I read a lot about pcr test and meaning of E and N numbers my results showed 34.45 and 34.65 apparently when they multiply your dna until they can find Covid most countries stop at 35 if not showing by then your result is negative I think Thailand go to 40.
Anyway I wish anyone with the misfortune to test positive either arriving or trying to leave a speedy recovery and be as lucky as myself