It's hard to say that's it's only teething pain, as that's an evaluation your physician would have to make. However, it is definitely possible that this is just the reaction to teething pain.
An 18-month-old doesn't have much exposure to pain in general, and when they're cutting teeth the pain can be both intense or enduring. It can interrupt their normal rest times (naps, sleep), which can further lower their tolerance. The combination of low exposure and low tolerance can make your child very unhappy and uncomfortable.
People will likely describe the child as "crabby". When it comes to parenting, it seems common for people to underplay the difficulties parents face. If we're going to be realistic, teething pain is probably the most pain an average infant will face, barring accidents. The closest my oldest came to similar pain was from a set of intramuscular immunization shots, but even that discomfort was temporary and tolerable.
To help deal with the pain, there are some more methods mentioned here that I would recommend: https://parenting.stackexchange.com/a/4714/11394
We had success with putting wet washcloths inside of plastic bags (like Ziploc), and freezing them, and also cold carrots. We also used infant Orajel when the discomfort was the worst.
If these methods help at all, then it is most likely just teething pain.
Another strategy is to provide the child some sugar water (sucrose water) for pain relief. This worked well for us sometimes. Other times we still had to use Tylenol (acetaminophen). On bad days, we'd usually end with a dose of Tylenol before bed to help sleeping be more successful. When an infant/toddlers awakens from pain in the middle of the night, they can be inconsolable and unwilling to accept anything to help pain relief.
Sugar water and Tylenol could also alleviate other pain, so they wouldn't help narrow down whether or not this is teething pain.
Another side-effect of teething pain could be hunger. If the child is uncomfortable while eating, they may eat less, and then end up hungry and uncomfortable, which could compound their pain. We tried focusing on cold-serve foods to help alleviate the pain and ensuring our son ate enough. (For us, there were times when it was evident he wasn't eating because of the pain, even though he was hungry.)
If you're truly concerned, make sure you're checking in with your child's physician, especially if you plan on using medicine regularly. You may not need an actual appointment, but just to give a call in. Our pediatrician was okay with it, and provided us a dosage for the Tylenol that was specific to our child. It was a lower dose than the minimum showed on the bottle. (In fact, the bottle should say to consult a physician for children under 2). They provided us this information over the phone.