Answer: In the name of Allah, Most Compassionate, Most Merciful,
Assalam’aleykum, I pray this finds you in the best of states.
Jazakum Allah khayr for your question!
No, your fast was not invalidated because you were unsure whether or not you have swallowed water at all in the first place and because even if you had swallowed water, you were unsure whether or not this swallowing happened after the entering of the Fajr time.
“Certainty is not lifted by a doubt.”
[Ibn Nujaym, al-Ashbah wa’l Nadha’ir; Majallat al-Ahkam al-`Adiliyya]
This means that if one is certain about something, such as the validity of our worship, with this being the basic assumption for all human actions, then we will keep assuming it valid until certain that it was not. Mere possibilities and even likelihoods do not change this.
And Allah knows best.
May Allah Most High grant you the best of this world and the next. Please keep us in your du’as!
[Ustadh] Sufyan Qufi