Yes, sounds like salt burn, which could be from too much fertilizer, or, depending on your water quality, could be salt from the irrigation water. Since the plant is in a container, do you get any leachate (water running out the drainage holes) when you water? If not, or if you keep the pot in a saucer to catch the leachate, which then is wicked back up into the pot later, even good-quality water will eventually build up toxic salt levels in the pot. I'd recommend, every 2nd or 3rd watering, taking the plant outdoors, removing the saucer, and watering with a large volume of water, to leach away excess salts. If it's from too much fertilizer, this will also help, but if there are granules of slow-release fertilizer accessable on the surface of the soil, you may want to scrape some of them away, before leaching.