Brief Answer:
as explained in details.

Detailed Answer:
Hi.
Thanks for your query.
To recapitulate: Female/28 - 30 weeks pregnancy - baby bump seems really big - hurting at the top of bump even on touching - since last week - single baby - placenta is anterior - wants to know the reason ...
Please give additional information so that I can assist you better:
Where exactly is the tenderness - on the uterus or at the abdominal wall?
Does the place of tenderness change from time to time or is stationary to one point?
When was the last ultrasound done?
Does coughing or straining for any reason increase the bump?

Since the uterus is not tender to touch, the probable causes can be:
Bump in the abdominal wall which can be a hernia - it will be stationary. pops on coughing and is reducible unless obstructed.
If the bump is in the abdominal wall, particularly in the rectus sheath area, it can be a desmoid tumor.
If the bump is on the uterus it can be fibroid.
Placental separation with hematoma formation, but could have been enlarging and causing far more problems within a week, hence least possible.

I would advise you the following:
First of all, consult your Gynecologist to get the correct clinical evaluation and examination to know what the cause may be and second opinion of a General Surgeon if there is nothing on the side of a Gynecologist.
Urgent Ultrasound of the abdominal wall, and whole abdomen, Uterus, its wall and the baby.
This will give the correct diagnosis.
I hope, this answer helps you, please feel free to ask for further relevant queries if you feel that there is a gap of communication.

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