Tray doesn't fit anymore. There is a gap between the tray and my lateral incisors, and sensitivity in my left canine. (Photo)

When you have space between the ends of your teeth and the aligners, that means those teeth are no longer tracking with the direction the aligners are taking the teeth. As you have said, as you progress to more aligners it will just get more noticeable. This is when Invisalign will allow the orthodontist to do a Refinement. This is where new impressions are taken, sent to the lab with instructions of what is to be done. You will then have new precisely fitting aligners to finish your treatment. Good luck.

If extrusion (downward movement) of your laterals was planned the gap between the aligner and your teeth may indicate the teeth are not "tracking" as planned. In some cases space can be planned into aligner stages to allow room for the tooth to move down into that new position. It is possible to bond a bracket on the laterals and utilize elastics (rubber bands) and pull the laterals down into position. Your orthodontist may feel it is best to finishing your current aligner set then proceed with a new "refinement" series of aligners to reestablish the planned movement(s). If you do not have buttons on the laterals at this time I would expect buttons (extrusion attachments) or elastics will be applied in the refinement stage. Every patient case is different so the best movement methods will be selected by your treating dentist. Upper laterals can be difficult teeth to treat and it is very common to see the situation illustrated in your photo. Be sure to do your part and wear your aligners exactly as the orthodontist/ dentist recommends. Transient pain like what you mentioned is common during any orthodontic treatment. You should discuss any discomfort that is a concern with your orthodontist/ dentist so they can inspect the area and give you the best assessment. It is very encouraging that you mentioned the pain has disappeared.Looks like you are well on the way to an excellent outcome.  We wish you the very best!!