Thanks for your reply.
See, as you must be knowing, all medicines can cause side effects.
Now, side effects can be dose dependent (occur while increasing the dose), dose independent or time dependent (may appear while starting the treatment or later while continuing the treatment).
One special type of side effects are idiosyncratic drug reactions which are not routinely seen but may occur rarely or unpredictably in some patients.
Diclofenac is a very commonly used analgesic worldwide. The type of side effect you experienced is rarely reported by patients taking it (I have never seen it in any of my patient till yet), so what you experienced could be taken as an idiosyncratic drug reaction in my view.
Every patient is unique and no one can predict in advance, how a particular patient is going to react to a particular medicine. There can be several side effects mentioned in the literature related to a particular medicine but it is not necessary that a patient taking that medicine is going to experience all of them. It is also possible that patient may not experience a single side effect or may experience mild transient side effect.
As I mentioned earlier, Gabapentin is a totally different medicine. It can be useful for neuropathic / chronic pain. It should not worsen mood symptoms or cause cause psychotic symptoms as seen clinically. In fact, we sometimes use it in our Bipolar patients and patients suffering from psychosis as an additional medicine for controlling the symptoms.
So you can discuss with your doctor about trying this medicine in your case. Hopefully, you will find it useful.
Please feel free to discuss further if you have any query regarding my answer.