Which option did you choose when you switched? If you switched online, you were given a choice to apply immediately, or backdate to the beginning of the cycle, or start it on the next billing cycle.
If you chose to apply immediately, then your texting for the first part of the billing cycle will be prorated and you may have overages. You said you were in the "middle of the month", so that would mean you would only be allowed half your normal text messages for the first part of the billing cycle. If you BACKDATED the change, then unlimited would apply to the whole cycle. And if you forward date it, then the unlimited would kick in for your next cycle.
If you are unsure, call customer service back to see when the unlimited option takes effect.