It looks like you have 37% flavors in the mix, which is very high for something like caramel, plus you've used a 'contaminated' bottle, and then you've mixed nearly 10ml up as the first batch of a trial mix.
None of these are a great idea
If you get 20 drops per ml then you have 3.5ml of flavoring:
4ml + 2ml + 3.5ml = 9.5ml
37% is flavor
It may be better to do it like this:
1. Clean any bottles thoroughly before use.
2. Mix ONLY 1ml of any new recipe to test it fully, before mixing large quantities.
3. Start with 5% flavor and work up. However some flavors, like menthol, are extremely powerful and you need to start at 1%.
4. It is very common to find that the flavor is different the next day, usually stronger, since it takes a little time for the mix to 'cook'.
Also note that something like a synthetic caramel flavor contains numerous chemical flavors, and usually a flavor booster that is similar in effect to MSG in food, and possibly preservatives, that may potentially be problematic for some people. In theory, these things are safe for most people, in low concentrations - but that is no guarantee that they will be OK for you, especially in high quantities such as you describe. 37% is very high indeed.
Clean the bottle, mix 1ml only, start with very little flavor and work up. If you get a problem immediately, then one of the base ingredients is a problem for you, not the flavoring.