It is true sinutab or sudafed will both probably raise blood sugar levels even if only temporarily.
Sympathomimetics (Includes Sinutab Regular Strength) ↔ diabetes
Moderate Potential Hazard, Moderate plausibility. Applies to: Diabetes Mellitus
Sympathomimetic agents may cause increases in blood glucose concentrations. These effects are usually transient and slight but may be significant with dosages higher than those normally recommended. Therapy with sympathomimetic agents should be administered cautiously in patients with diabetes mellitus. Closer monitoring of blood glucose concentrations may be appropriate.
In fact most of the over the counter cold remedies are not considered as very suitable for us.
Though tablets or capsules may be a better option than syrups. As they should contain less carbohydrates.
You would do better to discuss this with your pharmacist or doctor to get advise on what you should take.
I normally just take paracetamol or ibuprofen or sometimes both and go to bed.