Quote from angeldoll:

Amoxicillin. I have been on the pill off and on for years and I have never had this happen during an antibiotic before. Which is why I am a little concerned.

Some studies show that Amoxil can interact with the pill's effectiveness, as far as a method of contraception.

When did you have sex in correlation to taking the antibiotic? It's recommended that you use a back-up method for at least 7 days after taking the last antibiotic.

I still think that what you experienced is spotting, especially since you've just started Seasonale. Many women spot/bleed for several cycles on Seasonale. However, if it would calm your fears, take a pregnancy test.