Since you've been pregnant before it's highly unlikely that either you or hubby have any fertility issues and it likely would be a huge waste of $s to see someone about this. As someone else mentioned, it's likely stress, but you might consider that things happen for a reason and maybe it's better to wait. Honestly, having kids closer together is a mixed blessing - yes they can be closer, but they will also likely fight alot more. Your son would be just over 3 if you pregnant now (if your profile is accurate) and that time period often yields the worst sibling rivalry and resentment, as your son will be old enough to know he's been displaced. If you wait a bit longer you son will be mature enough and have far more appreciation for his sibling and you are still very, very young and still have LOTS of time to have LOTS of kids. My sister didn't start until she was 30 and had 10 kids, so be careful what you wish for!
Another perspective is whether you really want to be pregnant and be dealing with an almost 3 year old without your hubby around. I was a military brat and it's not easy across the board, so giving yourself and son a chance to enjoy yourselves as much as possible without either one of you (or your husband while he's away) having to worry about another child might be a very good thing.