Have any of your friends got bikes you could try just to compare??
The bike you bought, do you know if it was a man / woman that owned it and if it's a mens / ladies bike ... generally women's sit bones and therefore their saddles are wider than men's and men's bikes tend to have a longer reach than women's which would affect how you're positioned on the saddle.
As others have already said though a shop will be able to measure you although even with the right saddle it will still be a certain amount of time before you have no discomfort on longer rides.
In my younger years when I as extremely fit from other sports I had a mobile home at the coast, it closed for 4 months of the year and I wouldn't get on a bicycle in that time.
When the site re-opened for the summer I was easily able to ride 20 miles because of my fitness level but it would totally destroy my backside ... especially if I tried to sit on a bike in the next 2/3 days which was really painful and I've always assumed is just bruising.
A couple of weekends into the year and it stopped happening, the best way to avoid it altogether is to start with short rides often and build up the distance ... it'll only take a couple of weeks max.