What happened to my Bechamel sauce?!

Post your recipe.

You I don't think the liquid amount will be exact everytime. I add as I see fit.

And how high was your heat. It should be on a medium low. I also add the liquid a bt at a time, then whisk and let it thicken a bit and add more liquid and repeat. If it is getting thick right away that is a good sign that your heat is to hot.

Sounds like the liquid ratio for your recipe is off. Simple fix. Just add a little more liquid than the recipe calls for to start with. If it's still too thick keep adding a little liquid at a time until it reaches your desired consistency. Always allow the liquid to come to a boil before you add a little more because it's not until the liquid reaches a boil that you'll know if you need to add more liquid or not.

You're not done yet. How much milk have you put in? You need to add the milk a little at a time and stir it in well each time, letting it cook for about 30 seconds or so. Keep doing this until it's the consistency you want.

You needed to keep adding milk until it was thinned out.

Not enough milk

Sounds like too much flour.

too much flour and maybe turn your heat down