The dose makes the poison
I would wager a large bet that it was your 'cocktail' of the two drugs that caused your problem rather than something inherently bad about using either Sinuclear and Sudafed alone in the recommended amounts.
I'm not a proponent of 'popping pills' for everything that ails you, but those who suffer sinus blockage and ear problems during diving can often benefit from the right type and dose of a decongestant or antihistamine, depending upon the problem. However, I would not recommend experimenting with your own combinations unless you have the pharmacological training to recognize when you are combining two very similar medications that you can expect will have a compounded effect. That's what you did with your 'cocktail.'
There have been a number of studies published in the scientific and medical literature on the effects of various medications on athletic performance. As you might expect, few if any involve freediving. It is difficult to make broad generalizations about the results of those studies, however, I would say that in general, few drugs, if used at the recommended dosage, will be dangerous to you while freediving if you stay within reasonable limits (this is not to say that an elite competitor might not notice a significant difference when pushing his limits).
Of those drugs, however, pseudoephedrine, the active ingredient in Sudafed, speeds heart rate and blood pressure by causing vasoconstriction - the same effect that helps clear the sinuses. So I would avoid Sudafed if it affects you negatively. But honestly, you can't tell how it works for you when you combine it with other drugs in your own 'cocktail'.
There are other decongestants developed specifically to avoid affecting heartrate and blood pressure. My brother now uses one of these newer decongestants every time he freedives to prevent the blockage of his eustachian tubes that he otherwise experiences. He's now diving deeper and longer thanks to pharmacological treatment of his ear problems.