Sulfasalazine and low mood

Hi, I've experienced PsA for over 20 yes but only been diagnosed for 4 (probably a familiar story to some people) I have been taking MTX for around two years and now have the injection weekly. As some symptoms remained in some joints I have recently been prescribed Sulf on top of MTX,4 tablets a day, I am currently at 3 tableiof 500 working up to 4 and have noticedy mood is pretty low (as well as tiredness and headaches). I wondered if anyone else had experienced this whilst on sulfasalazine and if it subsidies? I can cope with the next day nausea of MTX but will drow the line at feeling miserable daily.