What is the exact location? What is the size of swelling? Is there any redness of the skin and area is slightly warm?
This looks an hair follicle infection. Boil and carbuncle are the other possibilities. You can try the following;
1) Warm sitz bath; three to four times a day
2) Application of the neosporin or betadine ointment (OTC, don't need prescription)
3) You can take Tylenol in pregnancy; this helps in reducing pain and inflammation.
If these measures do not help, you would need to go to an urgent care clinic for the prescription of the antibiotic.
Please feel free for your follow up questions.
This should not be connected with the straining. The straining may just made it predominant in symptoms.
It is pleasure assisting you.