Yup, normal sunburn caused by intense sunlight. Older leaves suffer the most since old leaves with old cells response slower to produce protein they need to cover theirself against UV heat. The result is dry dead cells on leaves like pic above.

One of the bottom leaves fell off. It was yellow. Is there anything I can do to help my baby get better? I am scared to death all the leaves will just fall off and die. Should I cut off the leaves with lots of dark spots?

Just keep eye on the healthy leaves. If other leaves stay healthy in 2 or 3 days, then nothing to worry. If the symptoms spread further, maybe soil nutrients deficiency is the cause.

No need to cut off any leaves since they still able to do photosynthesis, but if you think remove unhealthy leaves will make you feel better, then go ahead. Seems that you have enough healthy leaves to support plant growth.