Maybe not. Just because you have pressure in your sinus cavities does not mean an infection is present, over the counter sinus tabs can help releive this pressure or hot water in a bowl with some salt and breathing in the steam will help clear you up. If you have any green/yellow discharge after several days without relief then infection maybe possible and you should then consult your doctor.
Maybe. True bacterial sinus infections develop after 10-14 days of persistently "clogged" sinuses. If you can open the sinuses with antihistamines, nsaids, steroids, decongestants, saline flushes or osteopathic manipulation, you may be able to relieve the pressure and stave off a bacterial infection.
Possible. But sinus pressure can usually be relieved with otc decongestants. If the pressure persists for several days without relief, call your doctor..