If you call your clinic, they might say a little spotting could be an “implantation bleed”. In all honesty, we have no idea why some women spot a little bit; but if it only lasts a day or two, and is barely noticeable (pink or brown), it doesn’t seem to matter or affect your prognosis.
Success rates from FET cycles are in the 15-40% range for most clinics. I hope your cycle is a successful one!