Both of my babies ended up with acid reflux and for both of them the first signs showed up around 4-6 weeks of age. I would strongly encourage you to look up Infant GERD on the web and read about some of the symptoms. It took us several long weeks of trial and error with our first child before we figured out that this is what he had (which is actually quite common in infants). After figuring it out, and trying some various things at the doctor's suggestion, we ended up putting him on a small dose of acid reflux medicine (prevacid worked best for us, but there are others). After about a week of medicine, we had a whole new baby. :) His 'witching hour' went away completely.
With our second child, our daughter, when this started up around the same time, we knew immediately what we should try, and began doing those things. Eventually, she ended up on a mild dose of acid reflux medicine, as well, and once again, after about a week she was pretty happy most of the time.
I also want to note that while spitting up is a big symptom of infant GERD, NEITHER of my children were big spitter-uppers. I also will say that there are things you can try before resorting to medicine, because medicine in a small infant is obviously not something anyone wants to do unless other things are not working (the alternative is that the acid from baby's stomach is backing up in his/her esophagus, eroding it, and creating that painful sensation).
Your child may not have this...but like I said before, it is really common in infants, so you might just pay attention to the symptoms in case you notice any. Then let your doc know.
Another thing to note...(another can of worms to open)...people way back when used to never buckle up their children, and some of those people claim that there is no need to buckle up their (now) grandchildren, because they claim that their children were always fine. That doesn't necessarily mean that it is the best thing for the child. Since the "Back to Sleep" campaign began in 1994, which encouraged all babies to be put to sleep on their backs, the overall SIDS rates have dropped by OVER 50%. That is pretty powerful information in my personal opinion. I understand that some people choose to put their babies to sleep on their tummies, and many of those babies are just fine. I chose to put my babies to sleep on their backs, because I felt like the newest information on SIDS was pretty convincing. Once the baby is big enough to flip him/herself over to the tummy, then it's not such a big deal.
ADD: Both of my children went off acid reflux medicine around 12 months of age.