Uy, totally normal ang worry mo, lalo na first time, gets ko! Ubo at sipon habang pregnant ay common lang talaga, especially sa 18 weeks, pero the thing is kailangan mo talagang i-check with your OB asap kasi need nilang ma-assess kung ano ang cause (viral, allergy, infection) para malaman kung safe ang gamot for you at baby. Someone in my mum group had sipon na tumatagal ng ilang weeks at ang doc niya nag-prescribe ng pregnancy-safe cough syrup lang, so don't stress na walang options, marami!
The tricky part is hindi lahat ng over-the-counter meds okay sa buntis, kaya self-medicating is a no-no talaga. Your OB will know exactly what's safe at what's not for your stage. In the meantime, rest well (and hey, left side is actually better for pregnancy blood flow anyway, pero kung comfy ka sa right, don't overthink it, comfort matters too!), stay hydrated, at take it easy. Fingers crossed it's just a simple viral thing na mabilis mawala, but best to get your doctor's green light ASAP para makatulog ka ng mahimbing without the worry. You've got this!