If you are 15 dpo and no bbt drop but on 13 took a neg hpt did just test to early?

Yes, it is possible that you tested too early on 13 dpo. Implantation can occur anywhere between 6 and 12 dpo, and it can take several days for the hCG hormone to reach detectable levels in the urine. If you are still experiencing no BBT drop on 15 dpo, it is possible that implantation occurred later or that your hCG levels are rising more slowly than average. It is recommended to wait a few more days and test again to get a more accurate result.