That's very unlikely, but not impossible.
When you say HCG
tests, it means either urinary HCG kit/ card or blood test, what is called as serum beta HCG.
Urine tests detect pregnancy
a few days (3 to 5 days) later compared to blood test. It is because the pregnancy hormone, the so called beta HCG, gets produced by the placenta, secreted in the body to the blood stream and then gets excreted in the urine.
So, the first, more accurate and quantative (to know the rising titre
when repeated) is serum beta HCG compared to the urinary kit.
If serum beta HCG is in the negative range (practically speaking less than 50 units on the 31st day of regular cycles) or even urinary pregcard
test is negative (latest kits are very sensitive), one can safely assume that the woman is not pregnant
on that day.
The above tests may have to be repeated in case of doubt or women with irregular cycles.