This is not uncommon. In the future, start spraying the tree outside during late August and early September with Neem oil. Spray weekly for at least 4 weeks. I’d not suggest Neem Oil inside though. Now you best option will be Insecticidal soap spray. You’ll need to spray the plant weekly and very thoroughly paying attention to coating the entire surface of the plant with the spray. Concentrate on the undersides of the leaves. You’ll need to spray once a week for four or five weeks to get them under control and even then, they may crop back up from any that you missed during your spraying. The difficulty spraying indoors is a good reason to do outdoor sprayings outside a month or so before it comes inside whether you see insects on the plant or not. Once they come inside, the insect population explodes as you are now realizing.