Toilets bubble, shower drains slowly
I have two bathrooms in this house. We just bought it and moved in... for about a week everything worked perfectly. Then about 4-5 days ago we noticed the toilets started bubbling when one of us was taking a shower. I didn't think much of it. Well about 2 days after that the shower decided it didn't want to drain (well it does drain just at a very slow rate). Once the shower stopped draining it also stopped the toilets from flushing. They would just fill to the top and then about 2 hours later they would be back at the proper level (same as tub). As long as no one is showering the toilets work fine. One thing I find strange is that when the shower and toilet is backed up I can run the sink in the same bathroom and it runs / drains just fine. I have read many posts about how this could be a vent line problem or a main line clog problem (but if main line I'm curious why the sink doesn't back up). Any other light someone can shine on this about which of these they think the problem is and what steps I should do to address it? Thanks a lot in advance.