Taken from Breastfeeding and medications

"aspirin is an ingredient in many pain relieving products. Occasional or low doses (such as for blood thinning therapy) are safe to take while breastfeeding. However, if taken in large doses, aspirin may pass into the breast milk and cause adverse effects in your baby".

So have a dose and then see if you can get to the shops - paracetamol and ibuprofen are considered better options for longer term use.